netfilter: use fwmark_reflect in nf_send_reset

Otherwise, RST packets generated by ipt_REJECT always have mark 0 when
the routing is checked later in the same code path.

Fixes: e110861f8609 ("net: add a sysctl to reflect the fwmark on replies")
Cc: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: Pau Espin Pedrol <pau.espin@tessares.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/ipv4/netfilter/nf_reject_ipv4.c

@@ -126,6 +126,8 @@ void nf_send_reset(struct net *net, struct sk_buff *oldskb, int hook)
 	/* ip_route_me_harder expects skb->dst to be set */
 	skb_dst_set_noref(nskb, skb_dst(oldskb));
 
+	nskb->mark = IP4_REPLY_MARK(net, oldskb->mark);
+
 	skb_reserve(nskb, LL_MAX_HEADER);
 	niph = nf_reject_iphdr_put(nskb, oldskb, IPPROTO_TCP,
 				   ip4_dst_hoplimit(skb_dst(nskb)));

net/ipv6/netfilter/nf_reject_ipv6.c

@@ -157,6 +157,7 @@ void nf_send_reset6(struct net *net, struct sk_buff *oldskb, int hook)
 	fl6.fl6_sport = otcph->dest;
 	fl6.fl6_dport = otcph->source;
 	fl6.flowi6_oif = l3mdev_master_ifindex(skb_dst(oldskb)->dev);
+	fl6.flowi6_mark = IP6_REPLY_MARK(net, oldskb->mark);
 	security_skb_classify_flow(oldskb, flowi6_to_flowi(&fl6));
 	dst = ip6_route_output(net, NULL, &fl6);
 	if (dst->error) {
@@ -180,6 +181,8 @@ void nf_send_reset6(struct net *net, struct sk_buff *oldskb, int hook)
 
 	skb_dst_set(nskb, dst);
 
+	nskb->mark = fl6.flowi6_mark;
+
 	skb_reserve(nskb, hh_len + dst->header_len);
 	ip6h = nf_reject_ip6hdr_put(nskb, oldskb, IPPROTO_TCP,
 				    ip6_dst_hoplimit(dst));