Fix can_migrate to allow aggressive steal for idle cpus.  This -was- in
mainline, but I believe sched_domains kind of blasted it outta there.  IMO,
it's a no brainer for an idle cpu (with all that cache going to waste) to
be granted to steal a task.  The one enhancement I have made was to make
sure the whole cpu was idle.

Signed-off-by: <habanero@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>