sched, fair: Let minimally loaded cpu balance the group

Currently we let the leftmost (or first idle) cpu acend the
sched_domain tree and perform load-balancing. The result is that the
busiest cpu in the group might be performing this function and pull
more load to itself. The next load balance pass will then try to
equalize this again.

Change this to pick the least loaded cpu to perform higher domain
balancing.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
modified for Linux 3.0 from Linux 3.4+