Conntrack (struct nf_conn) gets pointer to netns: ->ct_net -- netns in which
it was created. It comes from netdevice.

->ct_net is write-once field.

Every conntrack in system has ->ct_net initialized, no exceptions.

->ct_net doesn't pin netns: conntracks are recycled after timeouts and
pinning background traffic will prevent netns from even starting shutdown
sequence.

Right now every conntrack is created in init_net.
Signed-off-by: NAlexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: NPatrick McHardy <kaber@trash.net>