perf/cgroup: Correct indirection in perf_less_group_idx()

The void* in perf_less_group_idx() is to a member in the array which points
at a perf_event*, as such it is a perf_event**.
Reported-By: John Sperbeck <jsperbeck@google.com>
Fixes: 6eef8a71 ("perf/core: Use min_heap in visit_groups_merge()")
Signed-off-by: Ian Rogers <irogers@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lkml.kernel.org/r/20200321164331.107337-1-irogers@google.com

kernel/events/core.c

@@ -3522,7 +3522,8 @@ static void cpu_ctx_sched_out(struct perf_cpu_context *cpuctx,
 
 static bool perf_less_group_idx(const void *l, const void *r)
 {
-	const struct perf_event *le = l, *re = r;
+	const struct perf_event *le = *(const struct perf_event **)l;
+	const struct perf_event *re = *(const struct perf_event **)r;
 
 	return le->group_index < re->group_index;
 }