block: turn Oops into WARNING if bdi_register_dev fails.

add_disk() does not return an error status, but it can still fail if there are problems elsewhere.

In particular a recent problem caused bdi_register_dev() to sometimes fail because the name was already in use.

In that case bdi->dev is NULL, so

	retval = sysfs_create_link(&disk_to_dev(disk)->kobj, &bdi->dev->kobj,
				   "bdi");

triggers an oops, after having already produced warnings.

This patch causes add_disk() to WARN and return if bdi_register_dev() fails, much like it already does if blk_alloc_devt() fails.

This should make no difference on a correctly functioning system, but can make a developers life a bit easier.

index 0a536dc05f3b..e351fc521053 100644
@@ -612,6 +612,10 @@ void add_disk(struct gendisk *disk)
 	/* Register BDI before referencing it from bdev */
 	bdi = &disk->queue->backing_dev_info;
 	bdi_register_dev(bdi, disk_devt(disk));
+	if (!bdi->dev) {
+		WARN_ON(1);
+		return;
+	}
 
 	blk_register_region(disk_devt(disk), disk->minors, NULL,
 			    exact_match, exact_lock, disk);

I guess that's the best workaround for now, but we'll really need to tackle the problem that add_disk can return errors proper eventually.