tcp: Respect SO_RCVLOWAT in tcp_poll().

Based upon a report by Vito Caputo.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp.c

@@ -384,13 +384,17 @@ unsigned int tcp_poll(struct file *file, struct socket *sock, poll_table *wait)
 
 	/* Connected? */
 	if ((1 << sk->sk_state) & ~(TCPF_SYN_SENT | TCPF_SYN_RECV)) {
+		int target = sock_rcvlowat(sk, 0, INT_MAX);
+
+		if (tp->urg_seq == tp->copied_seq &&
+		    !sock_flag(sk, SOCK_URGINLINE) &&
+		    tp->urg_data)
+			target--;
+
 		/* Potential race condition. If read of tp below will
 		 * escape above sk->sk_state, we can be illegally awaken
 		 * in SYN_* states. */
-		if ((tp->rcv_nxt != tp->copied_seq) &&
-		    (tp->urg_seq != tp->copied_seq ||
-		     tp->rcv_nxt != tp->copied_seq + 1 ||
-		     sock_flag(sk, SOCK_URGINLINE) || !tp->urg_data))
+		if (tp->rcv_nxt - tp->copied_seq >= target)
 			mask |= POLLIN | POLLRDNORM;
 
 		if (!(sk->sk_shutdown & SEND_SHUTDOWN)) {