netfilter: annotate nf_tables base hook ops

This will allow a followup patch to treat the 'ops->priv' pointer
as nft_chain argument without having to first walk the table/chains
to check if there is a matching base chain pointer.

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