sched: take rq lock prior to saving idle task's mark_start

When the idle task is being re-initialized during hotplug its
mark_start value must be retained. The runqueue lock must be
held when reading this value though to serialize this with
other CPUs that could update the idle task's window-based
statistics.

CRs-Fixed: 743991
Change-Id: I1bca092d9ebc32a808cea2b9fe890cd24dc868cd
Signed-off-by: Steve Muckle <smuckle@codeaurora.org>