memcg: avoid dangling reference count in creation failure.

When use_hierarchy is enabled, we acquire an extra reference count
in our parent during cgroup creation. We don't release it, though,
if any failure exist in the creation process.

Signed-off-by: Glauber Costa <glommer@parallels.com>
Reported-by: Michal Hocko <mhocko@suse>

mm/memcontrol.c

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 4f61e79..f1d6670 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6166,6 +6166,8 @@
 		 * call __mem_cgroup_free, so return directly
 		 */
 		mem_cgroup_put(memcg);
+		if (parent->use_hierarchy)
+			mem_cgroup_put(parent);
 	}
 	return error;
 }