fuzzing: Fix crypt2_load_fuzz to not touch const data
The crypt2_load_fuzz fuzzer needs to calculate LUKS2 header checksum to speed up fuzzing. Currently we incorrectly touch const data input.
This patch
- calculates only primary LUKS2 header checksum (ignores secondary header)
- uses temporary struct for modified data
- keps fuzzer going even with original data if checksum calc fails.
Hopefully solves unknown write issue in fuzzer (not real utils) on oss-fuzz.