tcp: Do not underestimate rwnd_limited

[ Upstream commit 41727549de3e7281feb174d568c6e46823db8684 ]

If available rwnd is too small, tcp_tso_should_defer()
can decide it is worth waiting before splitting a TSO packet.

This really means we are rwnd limited.

Fixes: 5615f886 ("tcp: instrument how long TCP is limited by receive window")
Signed-off-by: NEric Dumazet <edumazet@google.com>
Acked-by: NSoheil Hassas Yeganeh <soheil@google.com>
Reviewed-by: NYuchung Cheng <ycheng@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

net/ipv4/tcp_output.c

@@ -2338,9 +2338,12 @@ static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
 		} else {
 			if (!push_one &&
 			    tcp_tso_should_defer(sk, skb, &is_cwnd_limited,
-						 max_segs))
+						 max_segs)) {
+				if (!is_cwnd_limited)
+					is_rwnd_limited = true;
 				break;
 			}
+		}
 
 		limit = mss_now;
 		if (tso_segs > 1 && !tcp_urg_mode(tp))