[VLAN]: Handle vlan devices net namespace changing.

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: Pavel Emelyanov <xemul@openvz.org>
Acked-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>