[IPV4] ROUTE: Clean up proc files creation.

The rt_cache, stats/rt_cache and rt_acct(optional) files
creation looks a bit messy. Clean this out and join them
to other proc-related functions under the proper ifdef.

The struct net * argument in a new function will help net
namespaces patches look nicer.

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