perf report: Show zero counters as well in 'perf report --stat'

When recently using 'perf report --stat' it was not clear to me from the
output whether a particular statistics field (LOST_SAMPLES) was not
present, or just zero:

  fomalhaut:~> perf report --stat

  Aggregated stats:
           TOTAL events:     495984
            MMAP events:         85
            COMM events:       3389
            EXIT events:       1605
        THROTTLE events:          2
      UNTHROTTLE events:          2
            FORK events:       3377
          SAMPLE events:     472629
           MMAP2 events:      14753
  FINISHED_ROUND events:        139
      THREAD_MAP events:          1
         CPU_MAP events:          1
       TIME_CONV events:          1

I had to check the output several times to ascertain that I'm not
misreading the output, that the field didn't change and that I didn't
misremember the name. In fact I had to look into the perf source to make
sure that zero fields are indeed not shown.

With the patch applied:

  fomalhaut:~> perf report --stat

  Aggregated stats:
           TOTAL events:     495984
            MMAP events:         85
            LOST events:          0
            COMM events:       3389
            EXIT events:       1605
        THROTTLE events:          2
      UNTHROTTLE events:          2
            FORK events:       3377
            READ events:          0
          SAMPLE events:     472629
           MMAP2 events:      14753
             AUX events:          0
    ITRACE_START events:          0
    LOST_SAMPLES events:          0
          SWITCH events:          0
 SWITCH_CPU_WIDE events:          0
      NAMESPACES events:          0
            ATTR events:          0
      EVENT_TYPE events:          0
    TRACING_DATA events:          0
        BUILD_ID events:          0
  FINISHED_ROUND events:        139
        ID_INDEX events:          0
   AUXTRACE_INFO events:          0
        AUXTRACE events:          0
  AUXTRACE_ERROR events:          0
      THREAD_MAP events:          1
         CPU_MAP events:          1
     STAT_CONFIG events:          0
            STAT events:          0
      STAT_ROUND events:          0
    EVENT_UPDATE events:          0
       TIME_CONV events:          1
         FEATURE events:          0

It's pretty clear at a glance that LOST_SAMPLES is present but zero.

The original output can still be gotten via:

  fomalhaut:~> perf report --stat | grep -vw 0

  Aggregated stats:
           TOTAL events:     495984
            MMAP events:         85
            COMM events:       3389
            EXIT events:       1605
        THROTTLE events:          2
      UNTHROTTLE events:          2
            FORK events:       3377
          SAMPLE events:     472629
           MMAP2 events:      14753
  FINISHED_ROUND events:        139
      THREAD_MAP events:          1
         CPU_MAP events:          1
       TIME_CONV events:          1

So I don't think there's any real loss in functionality.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/20180307152430.7e5h7e657b7bgd7q@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>