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>