Enable optimization for nobounce_dmamap clients by setting the map
held by the client to NULL. This allows the macros in bus.h to check
against a constant to avoid function calls.
Don't attempt to 'free()' contigmalloced pages in bus_dmamem_free().
We currently leak these pages, which is not ideal, but is better than
a panic. The leak will be fixed when contigmalloc is merged into the
bus dma framework after 3.0R.
9aa4621326