af_unix: fix holding spinlock in oob handling

syzkaller found that OOB code was holding spinlock
while calling a function in which it could sleep.

Reported-by:  <syzbot+8760ca6c1ee783ac4abd@syzkaller.appspotmail.com>
Fixes: 314001f0

 ("af_unix: Add OOB support")
Signed-off-by: Rao Shoaib <rao.shoaib@oracle.com>
Link: https://lore.kernel.org/r/20210811220652.567434-1-Rao.Shoaib@oracle.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>