perf stat: Update walltime_nsecs_stats in interval mode

Some metrics (like GFLOPs) need walltime_nsecs_stats for each interval.
Compute it for each interval instead of only at the end.

Pointed out by Jiri.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: http://lkml.kernel.org/r/20170831194036.30146-12-andi@firstfloor.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>