tcp: lower congestion window on Fast Open SYNACK timeout

TCP sender would use congestion window of 1 packet on the second SYN
and SYNACK timeout except passive TCP Fast Open. This makes passive
TFO too aggressive and unfair during congestion at handshake. This
patch fixes this issue so TCP (fast open or not, passive or active)
always conforms to the RFC6298.

Note that tcp_enter_loss() is called only once during recurring
timeouts.  This is because during handshake, high_seq and snd_una
are the same so tcp_enter_loss() would incorrect set the undo state
variables multiple times.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_timer.c

@@ -393,6 +393,9 @@ static void tcp_fastopen_synack_timer(struct sock *sk)
 		tcp_write_err(sk);
 		return;
 	}
+	/* Lower cwnd after certain SYNACK timeout like tcp_init_transfer() */
+	if (icsk->icsk_retransmits == 1)
+		tcp_enter_loss(sk);
 	/* XXX (TFO) - Unlike regular SYN-ACK retransmit, we ignore error
 	 * returned from rtx_syn_ack() to make it more persistent like
 	 * regular retransmit because if the child socket has been accepted