netfilter: nf_tables: add hook ops to struct nft_pktinfo

Multi-family tables need the AF from the hook ops. Add a pointer to the
hook ops and replace usage of the hooknum member in struct nft_pktinfo.

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