net: send SYNACK packet with accepted fwmark

stable inclusion
from linux-4.19.199
commit d8142fffdb3e8c32b282a1a6c48b763eb92a8acf

--------------------------------

commit 43b90bfa upstream.

commit e05a90ec ("net: reflect mark on tcp syn ack packets")
fixed IPv4 only.

This part is for the IPv6 side.

Fixes: e05a90ec ("net: reflect mark on tcp syn ack packets")
Signed-off-by: NAlexander Ovechkin <ovov@yandex-team.ru>
Acked-by: NDmitry Yakunin <zeil@yandex-team.ru>
Reviewed-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

net/ipv6/tcp_ipv6.c

@@ -503,7 +503,8 @@ 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, np->tclass);
+		err = ip6_xmit(sk, skb, fl6, skb->mark ? : sk->sk_mark, opt,
+			       np->tclass);
 		rcu_read_unlock();
 		err = net_xmit_eval(err);
 	}