sched: Update shares on idle_balance

Since shares updates are no longer expensive and effectively local, update them
at idle_balance().  This allows us to more quickly redistribute shares to
another cpu when our load becomes idle.

Signed-off-by: Paul Turner <pjt@google.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20101115234938.204191702@google.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>