sched: refresh sched_clock() after acquiring rq lock in irq path

The wallclock time passed to sched_account_irqtime() may be stale
after we wait to acquire the runqueue lock. This could cause problems
in update_task_ravg because a different CPU may have advanced
this CPU's window_start based on a more up-to-date wallclock value,
triggering a BUG_ON(window_start > wallclock).

Change-Id: I316af62d1716e9b59c4a2898a2d9b44d6c7a75d8
Signed-off-by: Steve Muckle <smuckle@codeaurora.org>