libcryptsetup v2.0.1 introduced new API calls, supporting 64 bit wide
integers for `keyfile_offset`. This change invokes the new function
call, gets rid of the warning that was added in #7689, and removes
redundant #ifdefery and constant definitions.
See https://gitlab.com/cryptsetup/cryptsetup/issues/359.
Fixes#7677.
Jay Strict