The space calculation didn't include the terminating zero entry. That
ensured that the space for the netry would never be consumed. But the
remaining space was used to zero the end of the block so the final entry
wasn't being zeroed.
So have the space remaining include the terminating entry and factor
that into the space consumption of each entry being appended.
Signed-off-by: Zach Brown <zab@versity.com>