netfilter: ipset: list:set: fix reference counter update

The last element can be replaced or pushed off and in both
cases the reference counter must be updated.

Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/ipset/ip_set_list_set.c

@@ -174,9 +174,13 @@ list_set_add(struct list_set *map, u32 i, ip_set_id_t id,
 {
 	const struct set_elem *e = list_set_elem(map, i);
 
-	if (i == map->size - 1 && e->id != IPSET_INVALID_ID)
-		/* Last element replaced: e.g. add new,before,last */
-		ip_set_put_byindex(e->id);
+	if (e->id != IPSET_INVALID_ID) {
+		const struct set_elem *x = list_set_elem(map, map->size - 1);
+
+		/* Last element replaced or pushed off */
+		if (x->id != IPSET_INVALID_ID)
+			ip_set_put_byindex(x->id);
+	}
 	if (with_timeout(map->timeout))
 		list_elem_tadd(map, i, id, ip_set_timeout_set(timeout));
 	else