file_data: more bug files for hole-filling
file_data: more bug files for hole-filling

* Account for the bytes of a forced-dirty leaf block.
* Unreference the parent blocks of the shifted blocks.
* When reading from a hole, fill only to the start of the next block in the
  range.

Issue: KBFS-2091

keybase/client,keybase/kbfs,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/kbfs,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client