af_unix: Disable MSG_OOB handling for sockets in sockmap/sockhash

AF_UNIX socket tracks the most recent OOB packet (in its receive queue)
with an `oob_skb` pointer. BPF redirecting does not account for that: when
an OOB packet is moved between sockets, `oob_skb` is left outdated. This
results in a single skb that may be accessed from two different sockets.

Take the easy way out: silently drop MSG_OOB data targeting any socket that
is in a sockmap or a sockhash. Note that such silent drop is akin to the
fate of redirected skb's scm_fp_list (SCM_RIGHTS, SCM_CREDENTIALS).

For symmetry, forbid MSG_OOB in unix_bpf_recvmsg().

Fixes: 314001f0bf92 ("af_unix: Add OOB support")
Suggested-by: Kuniyuki Iwashima <[email protected]>
Signed-off-by: Michal Luczaj <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Tested-by: Jakub Sitnicki <[email protected]>
Reviewed-by: Kuniyuki Iwashima <[email protected]>
Reviewed-by: Jakub Sitnicki <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

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