cgroup: Fix issues in allow_attach callback

- Return -EINVAL when cgroups support isn't enabled
- Add allow_attach callback in CPU cgroups

Change-Id: Id3360b4a39919524fc4b6fcbd44fa2050009f000
Signed-off-by: Rom Lemarchand <romlem@android.com>

include/linux/cgroup.h

@@ -566,7 +566,7 @@ static inline int cgroup_init(void) { return 0; }
 static inline int subsys_cgroup_allow_attach(struct cgroup_subsys_state *css,
 					     struct cgroup_taskset *tset)
 {
-	return 0;
+	return -EINVAL;
 }
 #endif /* !CONFIG_CGROUPS */
 

kernel/sched/core.c

@@ -8609,6 +8609,7 @@ struct cgroup_subsys cpu_cgrp_subsys = {
 	.fork		= cpu_cgroup_fork,
 	.can_attach	= cpu_cgroup_can_attach,
 	.attach		= cpu_cgroup_attach,
+	.allow_attach   = subsys_cgroup_allow_attach,
 	.legacy_cftypes	= cpu_files,
 	.early_init	= 1,
 };