tcp: Fix recvmsg MSG_PEEK influence of blocking behavior.

Vito Caputo noticed that tcp_recvmsg() returns immediately from
partial reads when MSG_PEEK is used.  In particular, this means that
SO_RCVLOWAT is not respected.

Simply remove the test.  And this matches the behavior of several
other systems, including BSD.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp.c

@@ -1374,8 +1374,7 @@ int tcp_recvmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *msg,
 			    sk->sk_state == TCP_CLOSE ||
 			    (sk->sk_shutdown & RCV_SHUTDOWN) ||
 			    !timeo ||
-			    signal_pending(current) ||
-			    (flags & MSG_PEEK))
+			    signal_pending(current))
 				break;
 		} else {
 			if (sock_flag(sk, SOCK_DONE))