ovl: decode indexed dir file handles

Decoding an indexed dir file handle is done by looking up the file handle
in index dir by name and then decoding the upper dir from the index origin
file handle. The decoded upper path is used to lookup an overlay dentry of
the same path.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/overlayfs.h

@@ -270,6 +270,7 @@ int ovl_check_origin_fh(struct ovl_fs *ofs, struct ovl_fh *fh,
 			struct dentry *upperdentry, struct ovl_path **stackp);
 int ovl_verify_set_fh(struct dentry *dentry, const char *name,
 		      struct dentry *real, bool is_upper, bool set);
+struct dentry *ovl_index_upper(struct ovl_fs *ofs, struct dentry *index);
 int ovl_verify_index(struct ovl_fs *ofs, struct dentry *index);
 int ovl_get_index_name(struct dentry *origin, struct qstr *name);
 struct dentry *ovl_get_index_fh(struct ovl_fs *ofs, struct ovl_fh *fh);