Squashfs: refactor page_actor

This patch essentially does 3 things:
  1/ Always use an array of page to store the data instead of a mix of
     buffers and pages.
  2/ It is now possible to have 'holes' in a page actor, i.e. NULL
     pages in the array.
     When reading a block (default 128K), squashfs tries to grab all
     the pages covering this block. If a single page is up-to-date or
     locked, it falls back to using an intermediate buffer to do the
     read and then copy the pages in the actor. Allowing holes in the
     page actor remove the need for this intermediate buffer.
  3/ Refactor the wrappers to share code that deals with page actors.

Signed-off-by: Adrien Schildknecht <adriens@google.com>
Change-Id: I7991afb1e391fcbe62136bc1805352b7bc4985cd