FreeBSD Fsync on a directory has `Dir:false`

[tv42]

Directories are supposed to get a separate opcode, opFsyncdir.

https://github.com/bazillion/fuse/blob/00c7ac3b73bbaff171daab245982cac04456de72/fuse.go#L730

--- FAIL: TestFsyncDir (0.12s)
        mounted.go:119: FUSE: <- Init [ID=0x1 Node=0x0 Uid=1001 Gid=1001 Pid=3521] 7.8 ra=65536 fl=0
        mounted.go:119: FUSE: -> ID=0x1 Init {MaxReadahead:0 Flags:InitBigWrites MaxWrite:131072}
        mounted.go:119: FUSE: <- Open [ID=0x2 Node=0x1 Uid=1001 Gid=1001 Pid=3491] dir=true fl=OpenReadOnly
        mounted.go:119: FUSE: -> ID=0x2 Open {Handle:1 Flags:0}
        serve_test.go:1309: fsyncDir saw {Header:{Conn:<nil> ID:0 Node:0 Uid:0 Gid:0 Pid:0 msg:<nil>} Handle:0 Flags:0 Dir:false}, want {Header:{Conn:<nil> ID:0 Node:0 Uid:0 Gid:0 Pid:0 msg:<nil>} Handle:0 Flags:0 Dir:true}
        mounted.go:119: FUSE: <- Fsync [ID=0x3 Node=0x1 Uid=1001 Gid=1001 Pid=3491] Handle 1 Flags 1
        mounted.go:119: FUSE: -> ID=0x3
        mounted.go:119: FUSE: <- Release [ID=0x4 Node=0x1 Uid=1001 Gid=1001 Pid=3491] 0x1 fl=OpenReadOnly rfl=0 owner=0x0
        mounted.go:119: FUSE: -> ID=0x4
        mounted.go:119: FUSE: <- Forget [ID=0x5 Node=0x1 Uid=1001 Gid=1001 Pid=3491] 1
        mounted.go:119: FUSE: -> ID=0x5
        mounted.go:119: FUSE: <- Destroy [ID=0x6 Node=0x0 Uid=1001 Gid=1001 Pid=3491]
        mounted.go:119: FUSE: -> ID=0x6