The current dma_map_sg based implementation for bulk messages
computes many offsets into a single allocation multiple times in
both the create and free code paths. This is inefficient,
error prone and in fact still has a few lingering issues
with arm64.
This change replaces a small portion of that inplementation with
new code that uses a new struct vchiq_pagelist_info to store the
needed information rather then complex offset calculations.
This improved implementation should be more efficient and easier
to understand and maintain.