netfilter: nf_tables: export rule-set generation ID

This patch exposes the ruleset generation ID in three ways:

1) The new command NFT_MSG_GETGEN that exposes the 32-bits ruleset
   generation ID. This ID is incremented in every commit and it
   should be large enough to avoid wraparound problems.

2) The less significant 16-bits of the generation ID are exposed through
   the nfgenmsg->res_id header field. This allows us to quickly catch
   if the ruleset has change between two consecutive list dumps from
   different object lists (in this specific case I think the risk of
   wraparound is unlikely).

3) Userspace subscribers may receive notifications of new rule-set
   generation after every commit. This also provides an alternative
   way to monitor the generation ID. If the events are lost, the
   userspace process hits a overrun error, so it knows that it is
   working with a stale ruleset anyway.

Patrick spotted that rule-set transformations in userspace may take
quite some time. In that case, it annotates the 32-bits generation ID
before fetching the rule-set, then:

1) it compares it to what we obtain after the transformation to
   make sure it is not working with a stale rule-set and no wraparound
   has ocurred.

2) it subscribes to ruleset notifications, so it can watch for new
   generation ID.

This is complementary to the NLM_F_DUMP_INTR approach, which allows
us to detect an interference in the middle one single list dumping.
There is no way to explicitly check that an interference has occurred
between two list dumps from the kernel, since it doesn't know how
many lists the userspace client is actually going to dump.

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