tcp: extract code to compute SYNACK RTT

In preparation for adding another spot where we compute the SYNACK
RTT, extract this code so that it can be shared.
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/tcp.h

@@ -1137,6 +1137,15 @@ static inline void tcp_openreq_init(struct request_sock *req,
 	ireq->loc_port = tcp_hdr(skb)->dest;
 }
 
+/* Compute time elapsed between SYNACK and the ACK completing 3WHS */
+static inline void tcp_synack_rtt_meas(struct sock *sk,
+				       struct request_sock *req)
+{
+	if (tcp_rsk(req)->snt_synack)
+		tcp_valid_rtt_meas(sk,
+		    tcp_time_stamp - tcp_rsk(req)->snt_synack);
+}
+
 extern void tcp_enter_memory_pressure(struct sock *sk);
 
 static inline int keepalive_intvl_when(const struct tcp_sock *tp)

net/ipv4/tcp_ipv4.c

@@ -1733,9 +1733,7 @@ struct sock *tcp_v4_syn_recv_sock(struct sock *sk, struct sk_buff *skb,
 		newtp->advmss = tcp_sk(sk)->rx_opt.user_mss;
 
 	tcp_initialize_rcv_mss(newsk);
-	if (tcp_rsk(req)->snt_synack)
-		tcp_valid_rtt_meas(newsk,
-		    tcp_time_stamp - tcp_rsk(req)->snt_synack);
+	tcp_synack_rtt_meas(newsk, req);
 	newtp->total_retrans = req->retrans;
 
 #ifdef CONFIG_TCP_MD5SIG

net/ipv6/tcp_ipv6.c

@@ -1348,9 +1348,7 @@ static struct sock * tcp_v6_syn_recv_sock(struct sock *sk, struct sk_buff *skb,
 		newtp->advmss = tcp_sk(sk)->rx_opt.user_mss;
 
 	tcp_initialize_rcv_mss(newsk);
-	if (tcp_rsk(req)->snt_synack)
-		tcp_valid_rtt_meas(newsk,
-		    tcp_time_stamp - tcp_rsk(req)->snt_synack);
+	tcp_synack_rtt_meas(newsk, req);
 	newtp->total_retrans = req->retrans;
 
 	newinet->inet_daddr = newinet->inet_saddr = LOOPBACK4_IPV6;