rc: add srcFs and dstFs to core/stats and core/transferred stats

Before this change it wasn't possible to see where transfers were
going from and to in core/stats and core/transferred.

When use in rclone mount in particular this made interpreting the
stats very hard.

lib/http/serve/serve.go

@@ -79,7 +79,7 @@ func Object(w http.ResponseWriter, r *http.Request, o fs.Object) {
 		http.Error(w, http.StatusText(http.StatusNotFound), http.StatusNotFound)
 		return
 	}
-	tr := accounting.Stats(r.Context()).NewTransfer(o)
+	tr := accounting.Stats(r.Context()).NewTransfer(o, nil)
 	defer func() {
 		tr.Done(r.Context(), err)
 	}()