netfilter: nf_tables: add generation mask to tables

This patch addresses two problems:

1) The netlink dump is inconsistent when interfering with an ongoing
   transaction update for several reasons:

1.a) We don't honor the internal NFT_TABLE_INACTIVE flag, and we should
     be skipping these inactive objects in the dump.

1.b) We perform speculative deletion during the preparation phase, that
     may result in skipping active objects.

1.c) The listing order changes, which generates noise when tracking
     incremental ruleset update via tools like git or our own
     testsuite.

2) We don't allow to add and to update the object in the same batch,
   eg. add table x; add table x { flags dormant\; }.

In order to resolve these problems:

1) If the user requests a deletion, the object becomes inactive in the
   next generation. Then, ignore objects that scheduled to be deleted
   from the lookup path, as they will be effectively removed in the
   next generation.

2) From the get/dump path, if the object is not currently active, we
   skip it.

3) Support 'add X -> update X' sequence from a transaction.

After this update, we obtain a consistent list as long as we stay
in the same generation. The userspace side can detect interferences
through the generation counter so it can restart the dumping.

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