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

net/can/af_can.c

@@ -954,6 +954,7 @@ static struct notifier_block can_netdev_notifier __read_mostly = {
 static struct pernet_operations can_pernet_ops __read_mostly = {
 	.init = can_pernet_init,
 	.exit = can_pernet_exit,
+	.async = true,
 };
 
 static __init int can_init(void)