cryptocheck: Expand the set of sizes tested by -z.

Test individual sizes up to the max encryption block length as well as
a few sizes that include 1 full block and a partial block before
doubling the size.

Reviewed by: cem, markj
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D29518

(cherry picked from commit c86de1dab8e65bc9d11501ca51f2e152276cb94e)