net: Convert can_pernet_ops

These pernet_operations create and destroy /proc entries
and cancel per-net timer.

Also, there are unneed iterations over empty list of net
devices, since all net devices must be already moved
to init_net or unregistered by default_device_ops. This
already was mentioned here:

https://marc.info/?l=linux-can&m=150169589119335&w=2

So, it looks safe to make them async.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>