netfilter: nf_tables: use net_generic infra for transaction data

This moves all nf_tables pernet data from struct net to a net_generic
extension, with the exception of the gencursor.

The latter is used in the data path and also outside of the nf_tables
core. All others are only used from the configuration plane.

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