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>