crypto routines: Hint minimum buffer sizes to the compiler

Use the C99 'static' keyword to hint to the compiler IVs and output digest
sizes. The keyword informs the compiler of the minimum valid size for a given
array. Obviously not every pointer can be validated (i.e., the compiler can
produce false negative but not false positive reports).

No functional change. No ABI change.

Sponsored by: EMC / Isilon Storage Division