netfilter: nf_tables: variable sized set element keys / data

This patch changes sets to support variable sized set element keys / data
up to 64 bytes each by using variable sized set extensions. This allows
to use concatenations with bigger data items suchs as IPv6 addresses.

As a side effect, small keys/data now don't require the full 16 bytes
of struct nft_data anymore but just the space they need.

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