sched: more agressive idle balancing

the Linux scheduler is starving a number of workloads. So default
to more agressive idle-balancing. This hurts lmbench context-switching
numbers (which was the main reason we sucked at idle-balancing for
such a long time) but the lmbench numbers are fine once the system is
minimally utilized.

Signed-off-by: Ingo Molnar <mingo@elte.hu>