kernel: port another missing upstream change to xt_FLOWOFFLOAD on 4.19

Signed-off-by: Felix Fietkau <nbd@nbd.name>

target/linux/generic/hack-4.19/650-netfilter-add-xt_OFFLOAD-target.patch

@@ -98,7 +98,7 @@ Signed-off-by: Felix Fietkau <nbd@nbd.name>
  obj-$(CONFIG_NETFILTER_XT_TARGET_LED) += xt_LED.o
 --- /dev/null
 +++ b/net/netfilter/xt_FLOWOFFLOAD.c
-@@ -0,0 +1,383 @@
+@@ -0,0 +1,387 @@
 +/*
 + * Copyright (C) 2018 Felix Fietkau <nbd@nbd.name>
 + *
@@ -344,13 +344,13 @@ Signed-off-by: Felix Fietkau <nbd@nbd.name>
 +flowoffload_tg(struct sk_buff *skb, const struct xt_action_param *par)
 +{
 +	const struct xt_flowoffload_target_info *info = par->targinfo;
++	struct tcphdr _tcph, *tcph = NULL;
 +	enum ip_conntrack_info ctinfo;
 +	enum ip_conntrack_dir dir;
 +	struct nf_flow_route route;
 +	struct flow_offload *flow;
 +	struct nf_conn *ct;
 +	struct net *net;
-+	bool is_tcp = false;
 +
 +	if (xt_flowoffload_skip(skb, xt_family(par)))
 +		return XT_CONTINUE;
@@ -363,7 +363,11 @@ Signed-off-by: Felix Fietkau <nbd@nbd.name>
 +	case IPPROTO_TCP:
 +		if (ct->proto.tcp.state != TCP_CONNTRACK_ESTABLISHED)
 +			return XT_CONTINUE;
-+		is_tcp = true;
++
++		tcph = skb_header_pointer(skb, par->thoff,
++					  sizeof(_tcph), &_tcph);
++		if (unlikely(!tcph || tcph->fin || tcph->rst))
++			return XT_CONTINUE;
 +		break;
 +	case IPPROTO_UDP:
 +		break;
@@ -393,7 +397,7 @@ Signed-off-by: Felix Fietkau <nbd@nbd.name>
 +	if (!flow)
 +		goto err_flow_alloc;
 +
-+	if (is_tcp) {
++	if (tcph) {
 +		ct->proto.tcp.seen[0].flags |= IP_CT_TCP_FLAG_BE_LIBERAL;
 +		ct->proto.tcp.seen[1].flags |= IP_CT_TCP_FLAG_BE_LIBERAL;
 +	}