driver core: Allow device_move(dev, NULL).

If we allow NULL as the new parent in device_move(), we need to make sure
that the device is placed into the same place as it would if it was
newly registered:

- Consider the device virtual tree. In order to be able to reuse code,
  setup_parent() has been tweaked a bit.
- kobject_move() can fall back to the kset's kobject.
- sysfs_move_dir() uses the sysfs root dir as fallback.

Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Cc: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

lib/kobject.c

@@ -314,7 +314,7 @@ int kobject_rename(struct kobject * kobj, const char *new_name)
 /**
  *	kobject_move - move object to another parent
  *	@kobj:	object in question.
- *	@new_parent: object's new parent
+ *	@new_parent: object's new parent (can be NULL)
  */
 
 int kobject_move(struct kobject *kobj, struct kobject *new_parent)
@@ -330,8 +330,8 @@ int kobject_move(struct kobject *kobj, struct kobject *new_parent)
 		return -EINVAL;
 	new_parent = kobject_get(new_parent);
 	if (!new_parent) {
-		error = -EINVAL;
-		goto out;
+		if (kobj->kset)
+			new_parent = kobject_get(&kobj->kset->kobj);
 	}
 	/* old object path */
 	devpath = kobject_get_path(kobj, GFP_KERNEL);