sched: sched_clock() improvement: use in_nmi()

#include <linux/spinlock.h>

#include <linux/ktime.h>

#include <linux/module.h>

#include <linux/hardirq.h>

/*

 * Scheduler clock - returns current time in nanosec units.

	struct sched_clock_data *scd = cpu_sdc(cpu);

	u64 now, clock, this_clock, remote_clock;

	/*

	 * Normally this is not called in NMI context - but if it is,

	 * trying to do any locking here is totally lethal.

	 */

	if (unlikely(in_nmi()))

		return scd->clock;

	if (unlikely(!sched_clock_running))

		return 0ull;