sched: sched_clock() improvement: use in_nmi()

make sure we dont execute more complex sched_clock() code in NMI context.

Acked-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>