af_unix: Fix wrong ioctl(SIOCATMARK) when consumed OOB skb is at the head.

Even if OOB data is recv()ed, ioctl(SIOCATMARK) must return 1 when the
OOB skb is at the head of the receive queue and no new OOB data is queued.

Without fix:

  #  RUN           msg_oob.no_peek.oob ...
  # msg_oob.c:305:oob:Expected answ[0] (0) == oob_head (1)
  # oob: Test terminated by assertion
  #          FAIL  msg_oob.no_peek.oob
  not ok 2 msg_oob.no_peek.oob

With fix:

  #  RUN           msg_oob.no_peek.oob ...
  #            OK  msg_oob.no_peek.oob
  ok 2 msg_oob.no_peek.oob

Fixes: 314001f0 ("af_unix: Add OOB support")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>