ovl: fix encoding fid for lower only root

ovl_check_encode_origin() should return a positive number if the lower
dentry is to be encoded, zero otherwise.  If there's no upper layer at all
(read-only overlay), then it obviously needs to return positive.

This was broken by commit 16aac5ad ("ovl: support encoding
non-decodable file handles"), which didn't take the lower-only
configuration into account.

Fix by checking the no-upper-layer case up-front.
Reported-and-tested-by: Youzhong Yang <youzhong@gmail.com>
Closes: https://lore.kernel.org/all/CADpNCvaBimi+zCYfRJHvCOhMih8OU0rmZkwLuh24MKKroRuT8Q@mail.gmail.com/
Fixes: 16aac5ad ("ovl: support encoding non-decodable file handles")
Cc: <stable@vger.kernel.org> # v6.6
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/export.c

@@ -181,6 +181,10 @@ static int ovl_check_encode_origin(struct dentry *dentry)
 	struct ovl_fs *ofs = OVL_FS(dentry->d_sb);
 	bool decodable = ofs->config.nfs_export;
 
+	/* No upper layer? */
+	if (!ovl_upper_mnt(ofs))
+		return 1;
+
 	/* Lower file handle for non-upper non-decodable */
 	if (!ovl_dentry_upper(dentry) && !decodable)
 		return 1;
@@ -209,7 +213,7 @@ static int ovl_check_encode_origin(struct dentry *dentry)
 	 * ovl_connect_layer() will try to make origin's layer "connected" by
 	 * copying up a "connectable" ancestor.
 	 */
-	if (d_is_dir(dentry) && ovl_upper_mnt(ofs) && decodable)
+	if (d_is_dir(dentry) && decodable)
 		return ovl_connect_layer(dentry);
 
 	/* Lower file handle for indexed and non-upper dir/non-dir */