netfilter: core: support for NFPROTO_INET hook registration

Expand NFPROTO_INET in two hook registrations, one for NFPROTO_IPV4 and
another for NFPROTO_IPV6. Hence, we handle NFPROTO_INET from the core.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>