Problem was two race condtion in Aria page cache:
- find_block() didn't inform free_block() that it had released requests
- free_block() didn't handle pinned blocks, which could happen if
  free_block() was called as part of flush. This is fixed by not freeing
  blocks that are pinned.  This is safe as when maria_close() is called
  when last thread is using a table, there can be no pinned blocks. For
  other flush calls it's safe to ignore pinned blocks.