tcp: properly account Fast Open SYN-ACK retrans

Since the TFO socket is accepted right off SYN-data, the socket
owner can call getsockopt(TCP_INFO) to collect ongoing SYN-ACK
retransmission or timeout stats (i.e., tcpi_total_retrans,
tcpi_retransmits). Currently those stats are only updated
upon handshake completes. This patch fixes it.
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -5885,7 +5885,7 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb)
 		 * so release it.
 		 */
 		if (req) {
-			tp->total_retrans = req->num_retrans;
+			inet_csk(sk)->icsk_retransmits = 0;
 			reqsk_fastopen_remove(sk, req, false);
 		} else {
 			/* Make sure socket is routed, for correct metrics. */

net/ipv4/tcp_output.c

@@ -3568,6 +3568,8 @@ int tcp_rtx_synack(const struct sock *sk, struct request_sock *req)
 	if (!res) {
 		__TCP_INC_STATS(sock_net(sk), TCP_MIB_RETRANSSEGS);
 		__NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPSYNRETRANS);
+		if (unlikely(tcp_passive_fastopen(sk)))
+			tcp_sk(sk)->total_retrans++;
 	}
 	return res;
 }

net/ipv4/tcp_timer.c

@@ -384,6 +384,7 @@ static void tcp_fastopen_synack_timer(struct sock *sk)
 	 */
 	inet_rtx_syn_ack(sk, req);
 	req->num_timeout++;
+	icsk->icsk_retransmits++;
 	inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
 			  TCP_TIMEOUT_INIT << req->num_timeout, TCP_RTO_MAX);
 }