netfilter: conntrack: destroy kmemcache on module removal

I forgot to move the kmem_cache_destroy into the exit path.

Fixes: 0c5366b3 ("netfilter: conntrack: use single slab cache)
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_conntrack_core.c

@@ -1544,6 +1544,8 @@ void nf_conntrack_cleanup_end(void)
 	nf_conntrack_tstamp_fini();
 	nf_conntrack_acct_fini();
 	nf_conntrack_expect_fini();
+
+	kmem_cache_destroy(nf_conntrack_cachep);
 }
 
 /*