netfilter: nftables: Only run the nftables chains in the proper netns

- Register the nftables chains in the network namespace that they need
  to run in.

- Remove the hacks that stopped chains running in the wrong network
  namespace.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>