sched: core: reset preemption/irqsoff disable timestamp

Preempt/irqsoff tracer notes down preemption/irqs disabled timestamp
and compares it with enable timestamp to report prolonged disabling
callers at latency tracking. But they don't reset the time stamp at
re-enable time. The time stamp could be stale and possible to report
same latency multiple times or false positive. This could be possible
when callsites uses notrace API (__preempt_) for preemption disable
and tracable API (preempt_) for enable (ex: softirq).

Fix it by resetting the preempt/irqsoff disable timestamp at
re-enable time.

Change-Id: I6790227998881922a9339869ce9ee2dab6202567
Signed-off-by: Lingutla Chandrasekhar <clingutla@codeaurora.org>