[TCP]: Fix tcp_tso_should_defer() when limit>=65536

That's >= a full sized TSO frame, so we should always
return 0 in that case.

Based upon a report and initial patch from Lachlan
Andrew, final patch suggested by Herbert Xu.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_output.c

@@ -1036,6 +1036,10 @@ static int tcp_tso_should_defer(struct sock *sk, struct tcp_sock *tp, struct sk_
 
 	limit = min(send_win, cong_win);
 
+	/* If a full-sized TSO skb can be sent, do it. */
+	if (limit >= 65536)
+		return 0;
+
 	if (sysctl_tcp_tso_win_divisor) {
 		u32 chunk = min(tp->snd_wnd, tp->snd_cwnd * tp->mss_cache);