cgroup: allow ->post_create() to fail

There could be cases where controllers want to do initialization
operations which may fail from ->post_create().  This patch makes
->post_create() return -errno to indicate failure and online_css()
relay such failures.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Cc: Glauber Costa <glommer@parallels.com>
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index f4a9c98..03d8a92 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -440,7 +440,7 @@
 
 struct cgroup_subsys {
 	struct cgroup_subsys_state *(*create)(struct cgroup *cgrp);
-	void (*post_create)(struct cgroup *cgrp);
+	int (*post_create)(struct cgroup *cgrp);
 	void (*pre_destroy)(struct cgroup *cgrp);
 	void (*destroy)(struct cgroup *cgrp);
 	int (*can_attach)(struct cgroup *cgrp, struct cgroup_taskset *tset);