Comment 74 for bug 953875

It's a long-standing well-known limitation:

/* Options Debian's crypttab knows we don't:

    offset=
    skip=
    precheck=
    check=
    checkargs=
    noearly=
    loud=
    keyscript=
*/

Some of those will probably never be implemented (noearly, keyscript, loud, ...), but offset certainly should.