fix usage loss for snapshots with labels in MergeSnapshotter

Signed-off-by: liulanzheng <[email protected]>

Author: liulanzheng

snapshot/merge.go

@@ -185,19 +185,25 @@ func mergeUsageOf(info snapshots.Info) (usage snapshots.Usage, ok bool, rerr err
 	if info.Labels == nil {
 		return snapshots.Usage{}, false, nil
 	}
+	hasMergeUsageLabel := false
 	if str, ok := info.Labels[mergeUsageSizeLabel]; ok {
 		i, err := strconv.Atoi(str)
 		if err != nil {
 			return snapshots.Usage{}, false, err
 		}
 		usage.Size = int64(i)
+		hasMergeUsageLabel = true
 	}
 	if str, ok := info.Labels[mergeUsageInodesLabel]; ok {
 		i, err := strconv.Atoi(str)
 		if err != nil {
 			return snapshots.Usage{}, false, err
 		}
 		usage.Inodes = int64(i)
+		hasMergeUsageLabel = true
+	}
+	if !hasMergeUsageLabel {
+		return snapshots.Usage{}, false, nil
 	}
 	return usage, true, nil
 }