librbd: propagate area down to file_to_extents()

librbd: propagate area down to file_to_extents()

- readahead and PWL cache are limited to DATA area as explained in
  the previous commit
- DATA area is assumed for the journal as encryption can't be used
  with journaling anyway

To postpone the churn associated with passing area through
ImageDispatchInterface (where only WriteLogImageDispatch and
ImageDispatch care), add a new image dispatch flag.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>