sched: Fix race condition with active balance

There is a race condition between checking for whether an active load
balance request has been set and clearing the request. A cpu might have
an active load balance request set and queued but not executed yet.
Before the load balance request is executed the request flag might be
cleared by cpu isolation. Then subsequently the load balancer or tick
might try to do another active load balance.  This can cause the same
active load balance work to be queued twice causing report of list
corruption.

Fix this by moving the clearing of the request to the stopper thread and
ensuring that load balance will not try to queue a request on an
already isolated cpu.

Change-Id: I5c900d2ee161fa692d66e3e66012398869715662
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>