sched_ext: Replace naked scx_root dereferences in kobject callbacks

scx_attr_ops_show() and scx_uevent() access scx_root->ops.name directly.
This is problematic for two reasons:

1. The file-level comment explicitly identifies naked scx_root
   dereferences as a temporary measure that needs to be replaced
   with proper per-instance access.

2. scx_attr_events_show(), the neighboring sysfs show function in
   the same group, already uses the correct pattern:

       struct scx_sched *sch = container_of(kobj, struct scx_sched, kobj);

   Having inconsistent access patterns in the same sysfs/uevent
   group is error-prone.

The kobject embedded in struct scx_sched is initialized as:

    kobject_init_and_add(&sch->kobj, &scx_ktype, NULL, "root");

so container_of(kobj, struct scx_sched, kobj) correctly retrieves
the owning scx_sched instance in both callbacks.

Replace the naked scx_root dereferences with container_of()-based
access, consistent with scx_attr_events_show() and in preparation
for proper multi-instance scx_sched support.

Signed-off-by: zhidao su <suzhidao@xiaomi.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Author: zhidao su

kernel/sched/ext.c

@@ -3712,7 +3712,9 @@ static void scx_kobj_release(struct kobject *kobj)
 static ssize_t scx_attr_ops_show(struct kobject *kobj,
 				 struct kobj_attribute *ka, char *buf)
 {
-	return sysfs_emit(buf, "%s\n", scx_root->ops.name);
+	struct scx_sched *sch = container_of(kobj, struct scx_sched, kobj);
+
+	return sysfs_emit(buf, "%s\n", sch->ops.name);
 }
 SCX_ATTR(ops);
 
@@ -3756,7 +3758,9 @@ static const struct kobj_type scx_ktype = {
 
 static int scx_uevent(const struct kobject *kobj, struct kobj_uevent_env *env)
 {
-	return add_uevent_var(env, "SCXOPS=%s", scx_root->ops.name);
+	const struct scx_sched *sch = container_of(kobj, struct scx_sched, kobj);
+
+	return add_uevent_var(env, "SCXOPS=%s", sch->ops.name);
 }
 
 static const struct kset_uevent_ops scx_uevent_ops = {