netfilter: nf_tables: add support for dormant tables

This patch allows you to temporarily disable an entire table.
You can change the state of a dormant table via NFT_MSG_NEWTABLE
messages. Using this operation you can wake up a table, so their
chains are registered.

This provides atomicity at chain level. Thus, the rule-set of one
chain is applied at once, avoiding any possible intermediate state
in every chain. Still, the chains that belongs to a table are
registered consecutively. This also allows you to have inactive
tables in the kernel.

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