netfilter: nf_tables: convert hash and rbtree to set extensions

The set implementations' private struct will only contain the elements
needed to maintain the search structure, all other elements are moved
to the set extensions.

Element allocation and initialization is performed centrally by
nf_tables_api instead of by the different set implementations'
->insert() functions. A new "elemsize" member in the set ops specifies
the amount of memory to reserve for internal usage. Destruction
will also be moved out of the set implementations by a following patch.

Except for element allocation, the patch is a simple conversion to
using data from the extension area.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>