When van device is moved to another namespace proc files,
related to this device, should also change one.

Use the netdev REGISTER and UNREGISTER event handlers for this.
Signed-off-by: NPavel Emelyanov <xemul@openvz.org>
Acked-by: NPatrick McHardy <kaber@trash.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>