syzkaller hit a WARN_ON_ONCE(!scm->pid) in scm_pidfd_recv().
In unix_stream_read_generic(), if there is no skb in the queue, we could
bail out the do-while loop without calling scm_set_cred():
1. No skb in the queue
2. sk is non-blocking
or
shutdown(sk, RCV_SHUTDOWN) is called concurrently
or
peer calls close()
If the socket is configured with SO_PASSPIDFD, scm_pidfd_recv() would
populate cmsg with garbage emitting the warning.
Let's skip SCM_PIDFD if scm->pid is NULL in scm_pidfd_recv().
Note another way would be skip calling scm_recv() in such cases, but this
caused a regression resulting in commit 9d797ee2dce1 ("Revert "af_unix:
Call scm_recv() only after scm_set_cred()."").