[IPV4]: Use ctl paths to register net/ipv4/ table

This is the same as I did for the net/core/ table in the
second patch in his series: use the paths and isolate the
whole table in the .c file.

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