rbd: support reading parent page data

Currently, rbd_img_parent_read() assumes the incoming object request
contains bio data.  But if a layered image is part of a multi-layer
stack of images it will result in read requests of page data to parent
images.

Fortunately, it's not hard to add support for page data.

This resolves:
    http://tracker.ceph.com/issues/4939



Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>