netfilter: nf_tables: store and dump set policy

We want to know in which cases the user explicitly sets the policy
options. In that case, we also want to dump back the info.

Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>