af_unix: optimize unix_dgram_poll()

unix_dgram_poll() is pretty expensive to check POLLOUT status, because
it has to lock the socket to get its peer, take a reference on the peer
to check its receive queue status, and queue another poll_wait on
peer_wait. This all can be avoided if the process calling
unix_dgram_poll() is not interested in POLLOUT status. It makes
unix_dgram_recvmsg() faster by not queueing irrelevant pollers in
peer_wait.

On a test program provided by Alan Crequy :

Before:

real    0m0.211s
user    0m0.000s
sys     0m0.208s

After:

real    0m0.044s
user    0m0.000s
sys     0m0.040s
Suggested-by: NDavide Libenzi <davidel@xmailserver.org>
Reported-by: NAlban Crequy <alban.crequy@collabora.co.uk>
Acked-by: NDavide Libenzi <davidel@xmailserver.org>
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/unix/af_unix.c

@@ -2091,6 +2091,10 @@ static unsigned int unix_dgram_poll(struct file *file, struct socket *sock,
 			return mask;
 	}
 
+	/* No write status requested, avoid expensive OUT tests. */
+	if (wait && !(wait->key & (POLLWRBAND | POLLWRNORM | POLLOUT)))
+		return mask;
+
 	writable = unix_writable(sk);
 	other = unix_peer_get(sk);
 	if (other) {