rbd: fix error return code in rbd_dev_device_setup()

Fix to return -ENOMEM from the workqueue alloc error handling
case instead of 0, as done elsewhere in this function.

Reviewed-by: Alex Elder <[e-mail]>
Signed-off-by: Wei Yongjun <[e-mail]>

drivers/block/rbd.c

···
 	set_disk_ro(rbd_dev->disk, rbd_dev->mapping.read_only);
 
 	rbd_dev->rq_wq = alloc_workqueue("%s", 0, 0, rbd_dev->disk->disk_name);
-	if (!rbd_dev->rq_wq)
+	if (!rbd_dev->rq_wq) {
+		ret = -ENOMEM;
 		goto err_out_mapping;
+	}
 
 	ret = rbd_bus_add_dev(rbd_dev);
 	if (ret)