Use direct filesystem access for tar-split on aufs

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

daemon/graphdriver/aufs/aufs.go

@@ -367,6 +367,12 @@ func (a *Driver) Diff(id, parent string) (archive.Archive, error) {
 	})
 }
 
+// DiffPath returns path to the directory that contains files for the layer
+// differences. Used for direct access for tar-split.
+func (a *Driver) DiffPath(id string) (string, func() error, error) {
+	return path.Join(a.rootPath(), "diff", id), func() error { return nil }, nil
+}
+
 func (a *Driver) applyDiff(id string, diff archive.Reader) error {
 	dir := path.Join(a.rootPath(), "diff", id)
 	if err := chrootarchive.UntarUncompressed(diff, dir, &archive.TarOptions{