tcp: tcp_v[46]_conn_request: fix snt_synack initialization

Commit 016818d0 (tcp: TCP Fast Open Server - take SYNACK RTT after
completing 3WHS) changes the code to only take a snt_synack timestamp
when a SYNACK transmit or retransmit succeeds. This behaviour is later
broken by commit 843f4a55 (tcp: use tcp_v4_send_synack on first
SYN-ACK), as snt_synack is now updated even if tcp_v4_send_synack
fails.

Also, commit 3a19ce0e (tcp: IPv6 support for fastopen server) misses
the required IPv6 updates for 016818d0.

This patch makes sure that snt_synack is updated only when the SYNACK
trasnmit/retransmit succeeds, for both IPv4 and IPv6.

Cc: Cardwell <ncardwell@google.com>
Cc: Daniel Lee <longinus00@gmail.com>
Cc: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Octavian Purdila <octavian.purdila@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_ipv4.c

@@ -1370,7 +1370,6 @@ int tcp_v4_conn_request(struct sock *sk, struct sk_buff *skb)
 		goto drop_and_free;
 
 	tcp_rsk(req)->snt_isn = isn;
-	tcp_rsk(req)->snt_synack = tcp_time_stamp;
 	tcp_openreq_init_rwin(req, sk, dst);
 	fastopen = !want_cookie &&
 		   tcp_try_fastopen(sk, skb, req, &foc, dst);
@@ -1380,7 +1379,6 @@ int tcp_v4_conn_request(struct sock *sk, struct sk_buff *skb)
 		if (err || want_cookie)
 			goto drop_and_free;
 
-		tcp_rsk(req)->snt_synack = tcp_time_stamp;
 		tcp_rsk(req)->listener = NULL;
 		inet_csk_reqsk_queue_hash_add(sk, req, TCP_TIMEOUT_INIT);
 	}

net/ipv6/tcp_ipv6.c

@@ -497,6 +497,8 @@ static int tcp_v6_send_synack(struct sock *sk, struct dst_entry *dst,
 		skb_set_queue_mapping(skb, queue_mapping);
 		err = ip6_xmit(sk, skb, fl6, np->opt, np->tclass);
 		err = net_xmit_eval(err);
+		if (!tcp_rsk(req)->snt_synack && !err)
+			tcp_rsk(req)->snt_synack = tcp_time_stamp;
 	}
 
 done:
@@ -1100,7 +1102,6 @@ static int tcp_v6_conn_request(struct sock *sk, struct sk_buff *skb)
 		goto drop_and_free;
 
 	tcp_rsk(req)->snt_isn = isn;
-	tcp_rsk(req)->snt_synack = tcp_time_stamp;
 	tcp_openreq_init_rwin(req, sk, dst);
 	fastopen = !want_cookie &&
 		   tcp_try_fastopen(sk, skb, req, &foc, dst);