tun/netstack: fix race during `(*netTun).Close()`

[coadler]

During Close(), it's possible for WriteNotify() to race a channel send with the channel close. See the race below:

==================
WARNING: DATA RACE
Write at 0x00c000055450 by goroutine 18:
  runtime.closechan()
      /opt/hostedtoolcache/go/1.20.7/x64/src/runtime/chan.go:357 +0x0
  golang.zx2c4.com/wireguard/tun/netstack.(*netTun).Close()
      /home/runner/go/pkg/mod/golang.zx2c4.com/[email protected]/tun/netstack/tun.go:178 +0xde
  golang.zx2c4.com/wireguard/device.(*Device).Close()
      /home/runner/go/pkg/mod/golang.zx2c4.com/[email protected]/device/device.go:379 +0x181
  github.com/coder/wgtunnel/tunneld.(*API).Close()
      /home/runner/go/pkg/mod/github.com/coder/[email protected]/tunneld/tunneld.go:78 +0x64
  github.com/coder/coder/coderd/devtunnel_test.newTunnelServer.func2()
      /home/runner/actions-runner/_work/coder/coder/coderd/devtunnel/tunnel_test.go:211 +0x30
  testing.(*common).Cleanup.func1()
      /opt/hostedtoolcache/go/1.20.7/x64/src/testing/testing.go:1150 +0x193
  testing.(*common).runCleanup()
      /opt/hostedtoolcache/go/1.20.7/x64/src/testing/testing.go:1328 +0x1e9
  testing.tRunner.func2()
      /opt/hostedtoolcache/go/1.20.7/x64/src/testing/testing.go:1570 +0x52
  runtime.deferreturn()
      /opt/hostedtoolcache/go/1.20.7/x64/src/runtime/panic.go:476 +0x32
  testing.(*T).Run.func1()
      /opt/hostedtoolcache/go/1.20.7/x64/src/testing/testing.go:1629 +0x47

Previous read at 0x00c000055450 by goroutine 244:
  runtime.chansend()
      /opt/hostedtoolcache/go/1.20.7/x64/src/runtime/chan.go:160 +0x0
  golang.zx2c4.com/wireguard/tun/netstack.(*netTun).WriteNotify()
      /home/runner/go/pkg/mod/golang.zx2c4.com/[email protected]/tun/netstack/tun.go:165 +0xe6
  gvisor.dev/gvisor/pkg/tcpip/link/channel.(*queue).Write()
      /home/runner/go/pkg/mod/github.com/coder/[email protected]/pkg/tcpip/link/channel/channel.go:100 +0x183
  gvisor.dev/gvisor/pkg/tcpip/link/channel.(*Endpoint).WritePackets()
      /home/runner/go/pkg/mod/github.com/coder/[email protected]/pkg/tcpip/link/channel/channel.go:258 +0xb7
  gvisor.dev/gvisor/pkg/tcpip/stack.(*delegatingQueueingDiscipline).WritePacket()
      /home/runner/go/pkg/mod/github.com/coder/[email protected]/pkg/tcpip/stack/nic.go:146 +0x97
  gvisor.dev/gvisor/pkg/tcpip/stack.(*nic).writeRawPacket()
      /home/runner/go/pkg/mod/github.com/coder/[email protected]/pkg/tcpip/stack/nic.go:392 +0x84
  gvisor.dev/gvisor/pkg/tcpip/stack.(*nic).writePacket()
      /home/runner/go/pkg/mod/github.com/coder/[email protected]/pkg/tcpip/stack/nic.go:386 +0x59
  gvisor.dev/gvisor/pkg/tcpip/stack.(*nic).WritePacket()
      /home/runner/go/pkg/mod/github.com/coder/[email protected]/pkg/tcpip/stack/nic.go:347 +0x22b
  gvisor.dev/gvisor/pkg/tcpip/network/ipv6.(*endpoint).writePacket()
      /home/runner/go/pkg/mod/github.com/coder/[email protected]/pkg/tcpip/network/ipv6/ipv6.go:878 +0x408
  gvisor.dev/gvisor/pkg/tcpip/network/ipv6.(*endpoint).WritePacket()
      /home/runner/go/pkg/mod/github.com/coder/[email protected]/pkg/tcpip/network/ipv6/ipv6.go:829 +0x2d7
  gvisor.dev/gvisor/pkg/tcpip/stack.(*Route).WritePacket()
      /home/runner/go/pkg/mod/github.com/coder/[email protected]/pkg/tcpip/stack/route.go:495 +0xf8
  gvisor.dev/gvisor/pkg/tcpip/transport/tcp.sendTCP()
      /home/runner/go/pkg/mod/github.com/coder/[email protected]/pkg/tcpip/transport/tcp/connect.go:918 +0x3fb
  gvisor.dev/gvisor/pkg/tcpip/transport/tcp.(*endpoint).sendTCP()
      /home/runner/go/pkg/mod/github.com/coder/[email protected]/pkg/tcpip/transport/tcp/connect.go:816 +0x199
  gvisor.dev/gvisor/pkg/tcpip/transport/tcp.(*endpoint).sendRaw()
      /home/runner/go/pkg/mod/github.com/coder/[email protected]/pkg/tcpip/transport/tcp/connect.go:985 +0x53a
  gvisor.dev/gvisor/pkg/tcpip/transport/tcp.(*sender).sendSegmentFromPacketBuffer()
      /home/runner/go/pkg/mod/github.com/coder/[email protected]/pkg/tcpip/transport/tcp/snd.go:1686 +0x26d
  gvisor.dev/gvisor/pkg/tcpip/transport/tcp.(*sender).sendSegment()
      /home/runner/go/pkg/mod/github.com/coder/[email protected]/pkg/tcpip/transport/tcp/snd.go:1647 +0x386
  gvisor.dev/gvisor/pkg/tcpip/transport/tcp.(*sender).maybeSendSegment()
      /home/runner/go/pkg/mod/github.com/coder/[email protected]/pkg/tcpip/transport/tcp/snd.go:877 +0x704
  gvisor.dev/gvisor/pkg/tcpip/transport/tcp.(*sender).sendData()
      /home/runner/go/pkg/mod/github.com/coder/[email protected]/pkg/tcpip/transport/tcp/snd.go:981 +0x4fa
  gvisor.dev/gvisor/pkg/tcpip/transport/tcp.(*endpoint).sendData()
      /home/runner/go/pkg/mod/github.com/coder/[email protected]/pkg/tcpip/transport/tcp/connect.go:1009 +0xc4
  gvisor.dev/gvisor/pkg/tcpip/transport/tcp.(*endpoint).shutdownLocked()
      /home/runner/go/pkg/mod/github.com/coder/[email protected]/pkg/tcpip/transport/tcp/endpoint.go:2536 +0x44f
  gvisor.dev/gvisor/pkg/tcpip/transport/tcp.(*endpoint).closeLocked()
      /home/runner/go/pkg/mod/github.com/coder/[email protected]/pkg/tcpip/transport/tcp/endpoint.go:1063 +0xa9
  gvisor.dev/gvisor/pkg/tcpip/transport/tcp.(*endpoint).Close()
      /home/runner/go/pkg/mod/github.com/coder/[email protected]/pkg/tcpip/transport/tcp/endpoint.go:1033 +0x5d
  gvisor.dev/gvisor/pkg/tcpip/adapters/gonet.(*TCPConn).Close()
      /home/runner/go/pkg/mod/github.com/coder/[email protected]/pkg/tcpip/adapters/gonet/gonet.go:417 +0x4a
  github.com/coder/wgtunnel/tunneld.(*tracingConnWrapper).Close()
      /home/runner/go/pkg/mod/github.com/coder/[email protected]/tunneld/tracing.go:39 +0x7d
  net/http.(*persistConn).closeLocked()
      /opt/hostedtoolcache/go/1.20.7/x64/src/net/http/transport.go:2732 +0x222
  net/http.(*persistConn).readLoopPeekFailLocked()
      /opt/hostedtoolcache/go/1.20.7/x64/src/net/http/transport.go:2267 +0x344
  net/http.(*persistConn).readLoop()
      /opt/hostedtoolcache/go/1.20.7/x64/src/net/http/transport.go:2111 +0x1029
  net/http.(*Transport).dialConn.func5()
      /opt/hostedtoolcache/go/1.20.7/x64/src/net/http/transport.go:1765 +0x39