bank8k: allow for no zero base

bank8k: allow for no zero base

Tormod's patch worked by changing pagemap_realloc to pass top not size. In
fact we want to pass size so that

- We don't break any other banking code
- We can still sanely handle platforms with multiple processes per bank where
  pagemap_realloc has a real job. It's not quite that simple there are some
  other 32bit changes it would need, but lets not dig a hole to fall down