xen-blkfront: add error handling support for add_disk()

We never checked for errors on device_add_disk() as this function
returned void. Now that this is fixed, use the shiny new error
handling. The function xlvbd_alloc_gendisk() typically does the
unwinding on error on allocating the disk and creating the tag,
but since all that error handling was stuffed inside
xlvbd_alloc_gendisk() we must repeat the tag free'ing as well.

We set the info->rq to NULL to ensure blkif_free() doesn't crash
on blk_mq_stop_hw_queues() on device_add_disk() error as the queue
will be long gone by then.

Reviewed-by: Juergen Gross <[e-mail]>
Signed-off-by: Luis Chamberlain <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jens Axboe <[e-mail]>

drivers/block/xen-blkfront.c

···
 	for_each_rinfo(info, rinfo, i)
 		kick_pending_request_queues(rinfo);
 
-	device_add_disk(&info->xbdev->dev, info->gd, NULL);
+	err = device_add_disk(&info->xbdev->dev, info->gd, NULL);
+	if (err) {
+		blk_cleanup_disk(info->gd);
+		blk_mq_free_tag_set(&info->tag_set);
+		info->rq = NULL;
+		goto fail;
+	}
 
 	info->is_ready = 1;
 	return;