ovl: store layer index in ovl_layer

Store the fs root layer index inside ovl_layer struct, so we can
get the root fs layer index from merge dir lower layer instead of
find it with ovl_find_layer() helper.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h
index 9d0bc03..608e487 100644
--- a/fs/overlayfs/ovl_entry.h
+++ b/fs/overlayfs/ovl_entry.h
@@ -22,6 +22,8 @@ struct ovl_config {
 struct ovl_layer {
 	struct vfsmount *mnt;
 	dev_t pseudo_dev;
+	/* Index of this layer in fs root (upper == 0) */
+	int idx;
 };
 
 struct ovl_path {