net: send SYNACK packet with accepted fwmark

commit e05a90ec9e ("net: reflect mark on tcp syn ack packets")
fixed IPv4 only.

This part is for the IPv6 side.

Fixes: e05a90ec9e ("net: reflect mark on tcp syn ack packets")
Signed-off-by: Alexander Ovechkin <ovov@yandex-team.ru>
Acked-by: Dmitry Yakunin <zeil@yandex-team.ru>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/tcp_ipv6.c

@@ -555,7 +555,7 @@ static int tcp_v6_send_synack(const struct sock *sk, struct dst_entry *dst,
 		opt = ireq->ipv6_opt;
 		if (!opt)
 			opt = rcu_dereference(np->opt);
-		err = ip6_xmit(sk, skb, fl6, sk->sk_mark, opt,
+		err = ip6_xmit(sk, skb, fl6, skb->mark ? : sk->sk_mark, opt,
 			       tclass, sk->sk_priority);
 		rcu_read_unlock();
 		err = net_xmit_eval(err);