[IPV4] route: use read_mostly

The route table parameters are set based on system memory and sysctl
values that almost never change. Also the genid only changes every
10 minutes.

RTprint is defined by never used.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>