bugfix: updatedAt timestamp file may be empty

Signed-off-by: Michael Wan <zirenwan@gmail.com>

snapshots/overlay/overlay.go

@@ -168,7 +168,7 @@ func (o *snapshotter) Usage(ctx context.Context, key string) (snapshots.Usage, e
 	upperPath := o.upperPath(id)
 
 	if info.Kind == snapshots.KindActive {
-		du, err := fs.DiskUsage(upperPath)
+		du, err := fs.DiskUsage(ctx, upperPath)
 		if err != nil {
 			// TODO(stevvooe): Consider not reporting an error in this case.
 			return snapshots.Usage{}, err
@@ -225,7 +225,7 @@ func (o *snapshotter) Commit(ctx context.Context, name, key string, opts ...snap
 		return err
 	}
 
-	usage, err := fs.DiskUsage(o.upperPath(id))
+	usage, err := fs.DiskUsage(ctx, o.upperPath(id))
 	if err != nil {
 		return err
 	}