Fix deadlock in overlay snapshotter

The Usage implementation in the overlay driver allowed returning
before Rollback was called, leaving a transaction open and causing
further operations to deadlock. This could occur if the usage call
to the datastore returned an error, such as not found.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

snapshot/overlay/overlay.go

@@ -124,12 +124,13 @@ func (o *snapshotter) Usage(ctx context.Context, key string) (snapshot.Usage, er
 		return snapshot.Usage{}, err
 	}
 	id, info, usage, err := storage.GetInfo(ctx, key)
+	t.Rollback() // transaction no longer needed at this point.
+
 	if err != nil {
 		return snapshot.Usage{}, err
 	}
 
 	upperPath := o.upperPath(id)
-	t.Rollback() // transaction no longer needed at this point.
 
 	if info.Kind == snapshot.KindActive {
 		du, err := fs.DiskUsage(upperPath)