af_unix: use keyed wakeups

Instead of wakeup all sleepers, use wake_up_interruptible_sync_poll() to
wakeup only ones interested into writing the socket.

This patch is a specialization of commit 37e5540b3c (epoll keyed
wakeups: make sockets use keyed wakeups).

On a test program provided by Alan Crequy :

Before:
real    0m3.101s
user    0m0.000s
sys     0m6.104s

After:

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

Reported-by: Alban Crequy <alban.crequy@collabora.co.uk>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Davide Libenzi <davidel@xmailserver.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/unix/af_unix.c

@@ -316,7 +316,8 @@ static void unix_write_space(struct sock *sk)
 	if (unix_writable(sk)) {
 		wq = rcu_dereference(sk->sk_wq);
 		if (wq_has_sleeper(wq))
-			wake_up_interruptible_sync(&wq->wait);
+			wake_up_interruptible_sync_poll(&wq->wait,
+				POLLOUT | POLLWRNORM | POLLWRBAND);
 		sk_wake_async(sk, SOCK_WAKE_SPACE, POLL_OUT);
 	}
 	rcu_read_unlock();
@@ -1710,7 +1711,8 @@ static int unix_dgram_recvmsg(struct kiocb *iocb, struct socket *sock,
 		goto out_unlock;
 	}
 
-	wake_up_interruptible_sync(&u->peer_wait);
+	wake_up_interruptible_sync_poll(&u->peer_wait,
+					POLLOUT | POLLWRNORM | POLLWRBAND);
 
 	if (msg->msg_name)
 		unix_copy_addr(msg, skb->sk);