properly cancel context for streaming grpc

weed/pb/filer_pb/filer_client.go

@@ -85,11 +85,11 @@ func doList(filerClient FilerClient, fullDirPath util.FullPath, prefix string, f
 
 		glog.V(4).Infof("read directory: %v", request)
 		ctx, cancel := context.WithCancel(context.Background())
+		defer cancel()
 		stream, err := client.ListEntries(ctx, request)
 		if err != nil {
 			return fmt.Errorf("list %s: %v", fullDirPath, err)
 		}
-		defer cancel()
 
 		var prevEntry *Entry
 		for {