netfilter: arp_tables: pass table pointer via nf_hook_ops

Same change as previous patch.  Only difference:
no need to handle NULL template_ops parameter, the only caller
(arptable_filter) always passes non-NULL argument.

This removes all remaining accesses to net->ipv4.arptable_filter.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>