sched: window-stats: Avoid taking all cpu's rq->lock for long

reset_all_window_stats() walks task-list with all cpu's rq->lock held,
which can cause spinlock timeouts if task-list is huge (and hence lead
to a spinlock bug report). Avoid this by walking task-list without
cpu's rq->lock held.

Change-Id: Id09afd8b730fa32c76cd3bff5da7c0cd7aeb8dfb
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>