veritysetup: add support for hash-offset option

veritysetup: add support for hash-offset option

The verity parameter hash_area_offset allows to locate the superblock in
the hash device. It can be used to have a single device which contains
both data and hashes.

This adds the option hash-offset=BYTES (sixth argument) which is the
equivalent of the option --hash-offset in the veritysetup world.

See `veritysetup(8)` for more details.