/lib/modules should be mutable

The mount point of the modules filesystem can have new entries added to
it, so it should be mutable.  OverlayFS currently seems to ignore the
immutable attribute on the lower directory, but this might be an
OverlayFS bug, so don't rely on it.  The immutable attribute is honored
on subdirectories.

genfs.c

@@ -345,7 +345,7 @@ static int real_main(const char *const filesystem, const char *const uname) {
             .uname_or_label = uname,
         };
         process_dirent(fs, "", EXT2_ROOT_INO, 0, "",
-                       label_modules_object, mark_immutable);
+                       label_modules_object, false);
         if ((err = ext2fs_dir_iterate2(fs, EXT2_ROOT_INO, 0, NULL,
                                        root_iterate_callback, &data)))
             genfs_err(err, "processing /");