perf tool cgroup: initialize cgroup refcnt with refcount_set

Atomic reference counters were replaced by refcount_t in
commit 79c5fe6db8c7 ("perf/core: Fix error handling in perf_event_alloc()")

In util/cgroup.c atomic_inc was replaced by refcount_inc, but the latter is not mean to initiliaze refcounts with zero value. Add a path to initialize cgrp->refcnt == 0 using refcount_set.

Before this patch:

  $ perf stat -e cycles -C 0 -G /
  perf_before: /usr/local/.../tools/include/linux/refcount.h:108: refcount_inc: Assertion `!(!refcount_inc_not_zero(r))' failed.
  Aborted (core dumped)

After this patch:

  $ perf stat -e cycles -C 0 -G /
  Performance counter stats for 'CPU(s) 0':
     17,516,664      cycles                    /

---
 tools/perf/util/cgroup.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

index 03347748f3fa..7bbc19b3caf3 100644
@@ -133,7 +133,10 @@ static int add_cgroup(struct perf_evlist *evlist, char *str)
 
 	return -1;
 found:
-	refcount_inc(&cgrp->refcnt);
+	if (refcount_read(&cgrp->refcnt) == 0)
+		refcount_set(&cgrp->refcnt, 1);
+	else
+		refcount_inc(&cgrp->refcnt);
 	counter->cgrp = cgrp;
 	return 0;
 }

2.13.0.219.gdb65acc882-goog