vlan: assign PDE->data before gluing PDE into /proc tree

Simply replace proc_create and further data assigned with proc_create_data.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Acked-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>