net: send SYNACK packet with accepted fwmark

stable inclusion
from stable-5.10.53
commit b61d8814c4779f7810c96fa1d1d9e68fbd9d91df
bugzilla: 175574 https://gitee.com/openeuler/kernel/issues/I4DTUX

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b61d8814c4779f7810c96fa1d1d9e68fbd9d91df

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

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: NChen Jun <chenjun102@huawei.com>
Acked-by: NWeilong Chen <chenweilong@huawei.com>
Signed-off-by: NChen Jun <chenjun102@huawei.com>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>

net/ipv6/tcp_ipv6.c

@@ -540,7 +540,7 @@ 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);