netfilter: xt_hashlimit: fix race condition and simplify locking

As noticed by Shin Hong <hongshin@gmail.com>, there is a race between
htable_find_get() and htable_put():

htable_put():				htable_find_get():

					spin_lock_bh(&hashlimit_lock);
					<search entry>
atomic_dec_and_test(&hinfo->use)
					atomic_inc(&hinfo->use)
					spin_unlock_bh(&hashlimit_lock)
					return hinfo;
spin_lock_bh(&hashlimit_lock);
hlist_del(&hinfo->node);
spin_unlock_bh(&hashlimit_lock);
htable_destroy(hinfo);

The entire locking concept is overly complicated, tables are only
created/referenced and released in process context, so a single
mutex works just fine. Remove the hashinfo_spinlock and atomic
reference count and use the mutex to protect table lookups/creation
and reference count changes.

Signed-off-by: Patrick McHardy <kaber@trash.net>