netfilter: decnet: only register hooks in init namespace

looks like decnet isn't namespacified in first place, so restrict hook
registration to the initial namespace.

Prepares for eventual removal of legacy nf_register_hook() api.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>