tcp: skip cwnd moderation in TCP_CA_Open in tcp_try_to_open

The problem: Senders were overriding cwnd values picked during an undo
by calling tcp_moderate_cwnd() in tcp_try_to_open().

The fix: Don't moderate cwnd in tcp_try_to_open() if we're in
TCP_CA_Open, since doing so is generally unnecessary and specifically
would override a DSACK-based undo of a cwnd reduction made in fast
recovery.

Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -2881,7 +2881,8 @@ static void tcp_try_to_open(struct sock *sk, int flag)
 
 	if (inet_csk(sk)->icsk_ca_state != TCP_CA_CWR) {
 		tcp_try_keep_open(sk);
-		tcp_moderate_cwnd(tp);
+		if (inet_csk(sk)->icsk_ca_state != TCP_CA_Open)
+			tcp_moderate_cwnd(tp);
 	} else {
 		tcp_cwnd_down(sk, flag);
 	}