netfilter: nf_tables: select set backend flavour depending on description

netfilter: nf_tables: select set backend flavour depending on description

This patch adds the infrastructure to support several implementations of
the same set type. This selection will be based on the set description
and the features available for this set. This allow us to select set
backend implementation that will result in better performance numbers.

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