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

Simply replace proc_create and further data assigned with proc_create_data.
Additionally, there is no need to assign NULL to PDE->data after creation,
/proc generic has already done this for us.

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