sched: Fix idle balancing

An earlier commit reverts idle balancing throttling reset to fix a 30%
regression in volanomark throughput. We still need to reset idle_stamp
when we pull a task in newidle balance.

Reported-by: Alex Shi <alex.shi@intel.com>
Signed-off-by: Nikhil Rao <ncrao@google.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1290022924-3548-1-git-send-email-ncrao@google.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>