Argon2 draft defines suggested parameters for disk encryption use, but LUKS2
approach is slightly different. We need to provide platform independent
values. The values in draft expects 64bit systems (suggesting using 6 GiB
of RAM), while we need to provide compatibility with all 32bit systems,
so allocating more than 4GiB memory is not option for LUKS2.
The maximal limit in LUKS2 stays for 4 GiB, and by default LUKS2
PBKDF benchmarking sets maximum to 1 GIB, prefering increase of CPU cost.
But for the minimal memory cost we had a quite low limit 32 MiB.
This patch increases the bechmarking value to 64 MiB (as minimal
suggested values in Argon2 RFC). For compatibility reasons we still
allow older limit if set by a parameter.
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