netfilter: fix a use-after-free in mtype_destroy()

map->members is freed by ip_set_free() right before using it in
mtype_ext_cleanup() again. So we just have to move it down.

Reported-by: syzbot+4c3cc6dbe7259dbf9054@syzkaller.appspotmail.com
Fixes: 40cd63bf ("netfilter: ipset: Support extensions which need a per data destroy function")
Acked-by: Jozsef Kadlecsik <kadlec@netfilter.org>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/ipset/ip_set_bitmap_gen.h

@@ -60,9 +60,9 @@ mtype_destroy(struct ip_set *set)
 	if (SET_WITH_TIMEOUT(set))
 		del_timer_sync(&map->gc);
 
-	ip_set_free(map->members);
 	if (set->dsize && set->extensions & IPSET_EXT_DESTROY)
 		mtype_ext_cleanup(set);
+	ip_set_free(map->members);
 	ip_set_free(map);
 
 	set->data = NULL;