Do not assume buffer.View boundaries for packet headers

[ghananigans]

stack.PacketBuffer.Data is documented to be guaranteed not to contain headers that split across views. However, after a discussion with bhaskerh, this guarantee is not true. As such, we should remove all assumptions about buffer.View boundaries for packet headers.

        // Data holds the payload of the packet. For inbound packets, it also
	// holds the headers, which are consumed as the packet moves up the
	// stack. Headers are guaranteed not to be split across views.
	//
	// The bytes backing Data are immutable, but Data itself may be trimmed
	// or otherwise modified.
	Data buffer.VectorisedView

[ghananigans]

#2313 helps achieve this

[hbhasker]

Marking this closed. Will track further work around buffer reuse as separate tasks.

[github-actions[bot]]

There are TODOs still referencing this issue:

1. pkg/tcpip/header/parse/parse.go:53: Per RFC 791, IHL needs to be at least 5 in
2. pkg/tcpip/header/parse/parse.go:179: Figure out whether to reject this kind of

Search TODO

[github-actions[bot]]

A friendly reminder that this issue had no activity for 120 days.

[github-actions[bot]]

This issue has been closed due to lack of activity.

[github-actions[bot]]

There are TODOs still referencing this issue:

1. pkg/tcpip/header/parse/parse.go:52: Per RFC 791, IHL needs to be at least 5 in
2. pkg/tcpip/header/parse/parse.go:180: Figure out whether to reject this kind of

Search TODO

[github-actions[bot]]

A friendly reminder that this issue had no activity for 120 days.