ipv4: assign PDE->data before gluing PDE into /proc tree

ipv4: assign PDE->data before gluing PDE into /proc tree

The check for PDE->data != NULL becomes useless after the replacement
of proc_net_fops_create with proc_create_data.

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