af_unix: do not report POLLOUT on listeners

poll(POLLOUT) on a listener should not report fd is ready for
a write().

This would break some applications using poll() and pfd.events = -1,
as they would not block in poll()
Signed-off-by: NEric Dumazet <edumazet@google.com>
Reported-by: NAlan Burlison <Alan.Burlison@oracle.com>
Tested-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/unix/af_unix.c

@@ -326,9 +326,10 @@ static struct sock *unix_find_socket_byinode(struct inode *i)
 	return s;
 }
 
-static inline int unix_writable(struct sock *sk)
+static int unix_writable(const struct sock *sk)
 {
-	return (atomic_read(&sk->sk_wmem_alloc) << 2) <= sk->sk_sndbuf;
+	return sk->sk_state != TCP_LISTEN &&
+	       (atomic_read(&sk->sk_wmem_alloc) << 2) <= sk->sk_sndbuf;
 }
 
 static void unix_write_space(struct sock *sk)