From: Nick Piggin <nickpiggin@yahoo.com.au>

This patch starts to balance woken processes when half the relevant domain's
imbalance_pct is reached.  Previously balancing would start after a small,
constant difference in waker/wakee runqueue loads was reached, which would
cause too much process movement when there are lots of processes running.

It also turns wake balancing into a domain flag while previously it was always
on.  Now sched domains can "soft partition" an SMP system without using
processor affinities.