netfilter: xt_hashlimit: alloc hashtable with right size

struct xt_byteslimit_htable used hlist_head, but memory allocation is
done through sizeof(struct list_head).

Signed-off-by: Zhizhou Tian <zhizhou.tian@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>