vlan: Correctly handle device notifications for layered VLAN devices

Commit 30688a9a ([VLAN]: Handle vlan devices net namespace changing)
changed the device notifier to special-case notifications for VLAN
devices, effectively disabling state propagation to underlying VLAN
devices. This is needed for layered VLANs though, so restore the
original behaviour.

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