netfilter: xt_hashlimit: reduce hashlimit_mutex scope for htable_put()

It is unnecessary to hold hashlimit_mutex for htable_destroy()
as it is already removed from the global hashtable and its
refcount is already zero.

Also, switch hinfo->use to refcount_t so that we don't have
to hold the mutex until it reaches zero in htable_put().

Reported-and-tested-by:  <syzbot+adf6c6c2be1c3a718121@syzkaller.appspotmail.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>