sched: Drop active balance request upon cpu going offline

A cpu could mark its currently running task to be migrated to another
cpu (via rq->push_task/rq->push_cpu) and could go offline before
active load balance handles the request. In such case, clear the
active load balance request.

Change-Id: Ia3e668e34edbeb91d8559c1abb4cbffa25b1830b
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>