Also cache its value in active context, so we run benchmark
only once.

The patch also changes calculated value for LUKS1 key digest
to 125 miliseconds (it means that for full 8 used slots
the additional slow-down is circa 1 second).

Note that there is no need to have too high iteration count
for key digest; if it is too computationally expensive, attacker
will better decrypt of one sector with candidate key anyway.
(Check for a known signature.)

The reason to have some delay for key digest check was
to complicate brute-force search for volume key with LUKS header
only (and if RNG used to generate volumekey was flawed
allowing such a search i reasonable time).