Fix a long standing off-by-one bug in the softraid crypto number of keys

Fix a long standing off-by-one bug in the softraid crypto number of keys
calculation - we allow one key per 0.5TB, which should allow up to 16TB
disks, however the disk size was treated like a block offset and shifted.
This meant that the maximum size was actually 16TB minus one block.

While here also calculate the number of keys as an absolute value, rather
than as the upper inclusive bound - adjust the logic in the associated for
and if statements to match.

ok krw@ tb@