selftests/bpf: Test sockmap redirect for AF_UNIX MSG_OOB

Verify that out-of-band packets are silently dropped before they reach the
redirection logic.

The idea is to test with a 2 byte long send(). Should a MSG_OOB flag be in
use, only the last byte will be treated as out-of-band. Test fails if
verd_mapfd indicates a wrong number of packets processed (e.g. if OOB
wasn't dropped at the source) or if it was possible to recv() MSG_OOB from
the mapped socket, or if any stale OOB data have been left reachable from
the unmapped socket.

Signed-off-by: Michal Luczaj <mhal@rbox.co>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Jakub Sitnicki <jakub@cloudflare.com>
Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
Link: https://lore.kernel.org/bpf/20240713200218.2140950-5-mhal@rbox.co