rbd: allow null image name

Format 2 parent images are partially identified by their image id,
but it may not be possible to determine their image name.  The name
is not strictly needed for correct operation, so we won't be
treating it as an error if we don't know it.  Handle this case
gracefully in rbd_name_show().
Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>

drivers/block/rbd.c

@@ -1982,7 +1982,10 @@ static ssize_t rbd_name_show(struct device *dev,
 {
 	struct rbd_device *rbd_dev = dev_to_rbd_dev(dev);
 
+	if (rbd_dev->spec->image_name)
 		return sprintf(buf, "%s\n", rbd_dev->spec->image_name);
+
+	return sprintf(buf, "(unknown)\n");
 }
 
 static ssize_t rbd_image_id_show(struct device *dev,