BACKPORT: bpf: helper to pop data from messages

This adds a BPF SK_MSG program helper so that we can pop data from a
msg. We use this to pop metadata from a previous push data call.

Change-Id: Idcd8ce6393b152481de7d042994a795a10424bec
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>