veritysetup: add support for fec options

veritysetup: add support for fec options

The verity fec_* parameters allows to use Forward Error Correction to
recover from corruption if hash verification fails.

This adds the options fec_device, fec_offset and fec_roots (sixth
argument) which are the equivalent of the options --fec-device,
--fec-offset and --fec-roots in the veritysetup world.
 - fec-device=FILE
 - fec-offset=BYTES
 - fec-roots=UINT64

See `veritysetup(8)` for more details.