Merge branch 'af_unix-data-races'

Kuniyuki Iwashima says:

====================
af_unix: Fix four data-races.

While running syzkaller, KCSAN reported 3 data-races with
systemd-coredump using AF_UNIX sockets.

This series fixes the three and another one inspiered by
one of the reports.
====================

Signed-off-by: David S. Miller <davem@davemloft.net>