md: md.c: Return -ENODEV when mddev is NULL in rdev_attr_show

Commit c42d3240
("md: return -ENODEV if rdev has no mddev assigned") changed
rdev_attr_store to return -ENODEV when rdev->mddev is NULL, now do the
same to rdev_attr_show.
Signed-off-by: Marcos Paulo de Souza <marcos.souza.org@gmail.com>
Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/md.c

@@ -3356,7 +3356,7 @@ rdev_attr_show(struct kobject *kobj, struct attribute *attr, char *page)
 	if (!entry->show)
 		return -EIO;
 	if (!rdev->mddev)
-		return -EBUSY;
+		return -ENODEV;
 	return entry->show(rdev, page);
 }