netfilter: nf_tables: add NFTA_RULE_ID attribute

This new attribute allows us to uniquely identify a rule in transaction.
Robots may trigger an insertion followed by deletion in a batch, in that
scenario we still don't have a public rule handle that we can use to
delete the rule. This is similar to the NFTA_SET_ID attribute that
allows us to refer to an anonymous set from a batch.

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