MFC r282120:
The add_bounce_page() function can be called when loading physical
pages which pass a NULL virtual address. If the BUS_DMA_KEEP_PG_OFFSET
flag is set, use the physical address to compute the page offset
instead. The physical address should always be valid when adding
bounce pages and should contain the same page offset like the virtual
address.

Submitted by: Svatopluk Kraus <onwahe@gmail.com>
Reviewed by: jhb@