netfilter: nf_tables: enable conntrack if NAT chain is registered

Register conntrack hooks if the user adds NAT chains. Users get confused
with the existing behaviour since they will see no packets hitting this
chain until they add the first rule that refers to conntrack.

This patch adds new ->init() and ->free() indirections to chain types
that can be used by NAT chains to invoke the conntrack dependency.

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