Patch from Martin J. Bligh

This adds a small hook to the find_busiest_queue routine to allow us to
specify a mask of which CPUs to search over. In the NUMA case, it will
only balance inside the node (much cheaper to search, and stops tasks
from bouncing across nodes, which is very costly). The cpus_to_balance
routine is conditionally defined to ensure no impact to non-NUMA machines.

This is a tiny NUMA scheduler, but it needs the assistance of the second
and third patches in order to spread tasks across nodes.