rbd: clear ->xferred on error from rbd_obj_issue_copyup()

Otherwise the assert in rbd_obj_end_request() is triggered.

Fixes: 3da691bf43 ("rbd: new request handling code")
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

drivers/block/rbd.c

@@ -2585,6 +2585,7 @@ again:
 		ret = rbd_obj_issue_copyup(obj_req, obj_req->xferred);
 		if (ret) {
 			obj_req->result = ret;
+			obj_req->xferred = 0;
 			return true;
 		}
 		return false;