veritysetup-generator: use generic veritysetup writers at one more place

veritysetup-generator: use generic veritysetup writers at one more place

Let's use the common generator_write_veritysetup_unit_section(),
ggenerator_write_veritysetup_service_section(), generator_add_symlink()
implementation we already have at one more place.

This mostly generates the same unit, but for the first time hooks up
blockdev@dev-mapper-*.device for the device, which means things like
growfs on usr+root volumes will actually work now. (I mean, growfs
won#t, because verity devices are immutable after all, but things *like*
it that want to run between the device popping up and being mounted.)