af_unix: Support POLLPRI for OOB.

af_unix: Support POLLPRI for OOB.

BugLink: https://bugs.launchpad.net/bugs/1969110
commit d9a232d435dcc966738b0f414a86f7edf4f4c8c4 upstream.

The commit 314001f0bf92 ("af_unix: Add OOB support") introduced OOB for
AF_UNIX, but it lacks some changes for POLLPRI.  Let's add the missing
piece.

In the selftest, normal datagrams are sent followed by OOB data, so this
commit replaces `POLLIN | POLLPRI` with just `POLLPRI` in the first test
case.

Fixes: 314001f0bf92 ("af_unix: Add OOB support")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 21b6b8d43d87708c88ae34be67bb6cbba7bc60a0)
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>