netfilter: nf_tables: don't prevent event handler from device cleanup on netns exit

When a netnsamespace exits, the nf_tables pernet_ops will remove all rules.
However, there is one caveat:

Base chains that register ingress hooks will cause use-after-free:
device is already gone at that point.

The device event handlers prevent this from happening:
netns exit synthesizes unregister events for all devices.

However, an improper fix for a race condition made the notifiers a no-op
in case they get called from netns exit path, so revert that part.

This is safe now as the previous patch fixed nf_tables pernet ops
and device notifier initialisation ordering.

Fixes: 0a2cf5ee ("netfilter: nf_tables: close race between netns exit and rmmod")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>