netfilter: xt_TCPMSS: handle CHECKSUM_COMPLETE in tcpmss_tg6()

In case MSS option is added in TCP options, skb length increases by 4.
IPv6 needs to update skb->csum if skb has CHECKSUM_COMPLETE,
otherwise kernel complains loudly in netdev_rx_csum_fault() with a
stack dump.
Signed-off-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NPablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/xt_TCPMSS.c

@@ -228,7 +228,7 @@ tcpmss_tg6(struct sk_buff *skb, const struct xt_action_param *par)
 {
 	struct ipv6hdr *ipv6h = ipv6_hdr(skb);
 	u8 nexthdr;
-	__be16 frag_off;
+	__be16 frag_off, oldlen, newlen;
 	int tcphoff;
 	int ret;
 
@@ -244,7 +244,12 @@ tcpmss_tg6(struct sk_buff *skb, const struct xt_action_param *par)
 		return NF_DROP;
 	if (ret > 0) {
 		ipv6h = ipv6_hdr(skb);
-		ipv6h->payload_len = htons(ntohs(ipv6h->payload_len) + ret);
+		oldlen = ipv6h->payload_len;
+		newlen = htons(ntohs(oldlen) + ret);
+		if (skb->ip_summed == CHECKSUM_COMPLETE)
+			skb->csum = csum_add(csum_sub(skb->csum, oldlen),
+					     newlen);
+		ipv6h->payload_len = newlen;
 	}
 	return XT_CONTINUE;
 }