Fix TS-1707: Calculate blocks correctly on FreeBSD

FreeBSD calculation of

    blocks = size / hw_block_size;

is wrong, because *our* *store* block size defaults to 8k.

I'm further adding comments for future reference in other code paths.

TS-1707 further reminds us that these triple (or more) code paths
are confusing at best, and should be unified.
changes for TS-1707