The previous PBKDF2 benchmark code did not take into account
output key length.
For SHA1 (with 160-bits output) and 256-bit keys (and longer)
it means that the final value was higher than it should be.

For other hash algorithms (like SHA256 or SHA512) it caused
that iteration count was smaller (in comparison to SHA1) than
expected for the requested time period.

This patch fixes the code to use key size for the formatted device
(or default LUKS key size if running in informational benchmark mode).

Thanks to A.Visconti, S.Bossi, A.Calo and H.Ragab
(http://www.club.di.unimi.it/) for point this out.
(Based on "What users should know about Full Disk Encryption
based on LUKS" paper to be presented on CANS2015).