mtd_blkdevs: don't hold del_mtd_blktrans_dev in blktrans_{open, release}

There is nothing that this protects against except for slightly reducing
the window when new opens can appear just before calling del_gendisk.

Reported-by: Guenter Roeck <[e-mail]>
Signed-off-by: Christoph Hellwig <[e-mail]>
Tested-by: Guenter Roeck <[e-mail]>
Signed-off-by: Miquel Raynal <[e-mail]>
Link: https://lore.kernel.org/linux-mtd/[e-mail]

drivers/mtd/mtd_blkdevs.c

···
 	if (!dev)
 		return -ERESTARTSYS; /* FIXME: busy loop! -arnd*/
 
-	mutex_lock(&mtd_table_mutex);
 	mutex_lock(&dev->lock);
 
 	if (dev->open)
···
 unlock:
 	dev->open++;
 	mutex_unlock(&dev->lock);
-	mutex_unlock(&mtd_table_mutex);
 	blktrans_dev_put(dev);
 	return ret;
 
···
 	module_put(dev->tr->owner);
 	kref_put(&dev->ref, blktrans_dev_release);
 	mutex_unlock(&dev->lock);
-	mutex_unlock(&mtd_table_mutex);
 	blktrans_dev_put(dev);
 	return ret;
 }
···
 	if (!dev)
 		return;
 
-	mutex_lock(&mtd_table_mutex);
 	mutex_lock(&dev->lock);
 
 	if (--dev->open)
···
 	}
 unlock:
 	mutex_unlock(&dev->lock);
-	mutex_unlock(&mtd_table_mutex);
 	blktrans_dev_put(dev);
 }