netfilter: nft_objref: support for stateful object maps

This patch allows us to refer to stateful object dictionaries, the
source register indicates the key data to be used to look up for the
corresponding state object. We can refer to these maps through names or,
alternatively, the map transaction id. This allows us to refer to both
anonymous and named maps.

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