tcp: small optimization in tcp_v6_send_check()

For TCP flows, inet6_sk(sk)->saddr has the same value
than sk->sk_v6_rcv_saddr.

Using sk->sk_v6_rcv_saddr increases data locality.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/tcp_ipv6.c

@@ -1893,9 +1893,7 @@ static struct timewait_sock_ops tcp6_timewait_sock_ops = {
 
 INDIRECT_CALLABLE_SCOPE void tcp_v6_send_check(struct sock *sk, struct sk_buff *skb)
 {
-	struct ipv6_pinfo *np = inet6_sk(sk);
-
-	__tcp_v6_send_check(skb, &np->saddr, &sk->sk_v6_daddr);
+	__tcp_v6_send_check(skb, &sk->sk_v6_rcv_saddr, &sk->sk_v6_daddr);
 }
 
 const struct inet_connection_sock_af_ops ipv6_specific = {