netfilter: xtables: allow table definitions not backed by hook_ops

The ip(6)tables nat table is currently receiving skbs from the netfilter
core, after a followup patch skbs will be coming from the netfilter nat
core instead, so the table is no longer backed by normal hook_ops.

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