um: track 'parent' device in a local variable

In preparation for the removal of 'driverfs_dev' from 'struct gendisk'
use a local variable to track the parented vs un-parented case in
ubd_disk_register().

Cc: Jeff Dike <jdike@addtoit.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

arch/um/drivers/ubd_kern.c

@@ -801,6 +801,7 @@ static void ubd_device_release(struct device *dev)
 static int ubd_disk_register(int major, u64 size, int unit,
 			     struct gendisk **disk_out)
 {
+	struct device *parent = NULL;
 	struct gendisk *disk;
 
 	disk = alloc_disk(1 << UBD_SHIFT);
@@ -823,12 +824,12 @@ static int ubd_disk_register(int major, u64 size, int unit,
 		ubd_devs[unit].pdev.dev.release = ubd_device_release;
 		dev_set_drvdata(&ubd_devs[unit].pdev.dev, &ubd_devs[unit]);
 		platform_device_register(&ubd_devs[unit].pdev);
-		disk->driverfs_dev = &ubd_devs[unit].pdev.dev;
+		parent = &ubd_devs[unit].pdev.dev;
 	}
 
 	disk->private_data = &ubd_devs[unit];
 	disk->queue = ubd_devs[unit].queue;
-	add_disk(disk);
+	device_add_disk(parent, disk);
 
 	*disk_out = disk;
 	return 0;