netns xfrm: xfrm6_tunnel in netns

I'm not sure about rcu stuff near kmem cache destruction:
* checks for non-empty hashes look bogus, they're done _before_
  rcu_berrier()
* unregistering netns ops is done before kmem_cache destoy
  (as it should), and unregistering involves rcu barriers by itself

So it looks nothing should be done.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>