[IPV4] ROUTE: Collect proc-related functions together

The net/ipv4/route.c file declares some entries for proc
to dump some routing info. The reading functions are
scattered over this file - collect them together.

Besides, remove a useless IP_RT_ACCT_CPU macro.

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