netfilter: nf_tables: add reject module for NFPROTO_INET

Add a reject module for NFPROTO_INET. It does nothing but dispatch
to the AF-specific modules based on the hook family.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>