mtd: fix lock hierarchy in deregister_mtd_blktrans

There is a lock hierarchy of major_names_lock --> mtd_table_mutex. One
existing chain is as follows:

1. major_names_lock --> loop_ctl_mutex (when blk_request_module calls
loop_probe)

2. loop_ctl_mutex --> bdev->bd_mutex (when loop_control_ioctl calls
loop_remove, which then calls del_gendisk)

3. bdev->bd_mutex --> mtd_table_mutex (when blkdev_get_by_dev calls
__blkdev_get, which then calls blktrans_open)

Since unregister_blkdev grabs the major_names_lock, we need to call it
outside the critical section for mtd_table_mutex, otherwise we invert
the lock hierarchy.
Reported-by: Hillf Danton <hdanton@sina.com>
Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210717100719.728829-1-desmondcheongzx@gmail.com

drivers/mtd/mtd_blkdevs.c

@@ -561,8 +561,8 @@ int deregister_mtd_blktrans(struct mtd_blktrans_ops *tr)
 	list_for_each_entry_safe(dev, next, &tr->devs, list)
 		tr->remove_dev(dev);
 
-	unregister_blkdev(tr->major, tr->name);
 	mutex_unlock(&mtd_table_mutex);
+	unregister_blkdev(tr->major, tr->name);
 
 	BUG_ON(!list_empty(&tr->devs));
 	return 0;