hurd: Fix FS_RETRY_MAGICAL "machtype" handling

We need to set file_name, not update retryname. This is what the other
branches do.

Before this change, any attempt to access such a file would segfault due
to file_name being unset:

$ settrans -ac /tmp/my-machtype /hurd/magic machtype
$ cat /tmp/my-machtype
Segmentation fault

Checked on i686-gnu.

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>
Message-Id: <20230429131354.2507443-7-bugaevc@gmail.com>