librbd: pass area to ImageDispatchSpec::create_*()

librbd: pass area to ImageDispatchSpec::create_*()

- DATA area is assumed at the API layer as there is no way to pass
  an area
- DATA area is assumed by ImageWriteback because PWL cache persists
  image extents as provided by the user without any kind of designator
  and therefore can be active only in either area
- luks::FlattenRequest operates on CRYPTO_HEADER area

The passed area is acted upon in ImageDispatchSpec constructor in the
next commit.

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