Handle d_type == DT_UNKNOWN correctly

Handle d_type == DT_UNKNOWN correctly

As documented in the man-page, readdir() may return a directory entry with
d_type == DT_UNKNOWN. This must be handled for regular filesystems.

dirent_ensure_type() is available to set d_type if necessary. Use it in
some more places.

Without this systemd will fail to boot correctly with nfsroot and some
other filesystems.

Closes #13609