netfilter: arp_tables: register table in initns

arptables is broken since we didn't register the table anymore --
even 'arptables -L' fails.

Fixes: b9e69e12 ("netfilter: xtables: don't hook tables by default")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>