flow: better memory management

Allocate hash tables for every online cpus, not every possible ones.

NUMA aware allocations.

Dont use a full page on arches where PAGE_SIZE > 1024*sizeof(void *)

misc:
  __percpu , __read_mostly, __cpuinit annotations
  flow_compare_t is just an "unsigned long"

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>