rbd: fix an incorrect assertion condition

In rbd_img_obj_parent_read_full_callback() there is an assertion
intended to verify the size of the image request for a full parent
read was the size of the original request's target object.  But
assertion was looking at the parent image order rather than the
original one, and these values can differ.

Fix that.

This resolves:
    http://tracker.ceph.com/issues/4938



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