perf evsel: Add read_counter()

Add perf_evsel__read_counter() to read single or group counter. After
calling this function the counter's evsel::counts struct is filled with
values for the counter and member of its group if there are any.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/20170726120206.9099-3-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
diff --git a/tools/perf/util/stat.h b/tools/perf/util/stat.h
index 7522bf1..eacaf95 100644
--- a/tools/perf/util/stat.h
+++ b/tools/perf/util/stat.h
@@ -28,8 +28,9 @@ enum perf_stat_evsel_id {
 };
 
 struct perf_stat_evsel {
-	struct stats		res_stats[3];
-	enum perf_stat_evsel_id	id;
+	struct stats		 res_stats[3];
+	enum perf_stat_evsel_id	 id;
+	u64			*group_data;
 };
 
 enum aggr_mode {