scsi: sr: Add error handling support for add_disk()

We never checked for errors on add_disk() as this function returned
void. Now that this is fixed, use the shiny new error handling.

Just put the cdrom kref and have the unwinding be done by
sr_kref_release().

Link: https://lore.kernel.org/r/[e-mail]
Reviewed-by: Christoph Hellwig <[e-mail]>
Acked-by: Martin K. Petersen <[e-mail]>
Signed-off-by: Luis Chamberlain <[e-mail]>
Signed-off-by: Martin K. Petersen <[e-mail]>

drivers/scsi/sr.c

···
 	dev_set_drvdata(dev, cd);
 	disk->flags |= GENHD_FL_REMOVABLE;
 	sr_revalidate_disk(cd);
-	device_add_disk(&sdev->sdev_gendev, disk, NULL);
+
+	error = device_add_disk(&sdev->sdev_gendev, disk, NULL);
+	if (error) {
+		kref_put(&cd->kref, sr_kref_release);
+		goto fail;
+	}
 
 	sdev_printk(KERN_DEBUG, sdev,
 		    "Attached scsi CD-ROM %s\n", cd->cdi.name);