[NET]: NET_CLS_ROUTE : convert ip_rt_acct to per_cpu variables

ip_rt_acct needs 4096 bytes per cpu to perform some accounting.
It is actually allocated as a single huge array [4096*NR_CPUS]
(rounded up to a power of two)

Converting it to a per cpu variable is wanted to :
 - Save space on machines were num_possible_cpus() < NR_CPUS
 - Better NUMA placement (each cpu gets memory on its node)

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>