r269214, r269215, r269216, r269217, r269221:

busdma-v6 improvements, primarily:
- Allocate the temporary segments array per-map rather than per-tag.
- Avoid needlessly bouncing IO for mbufs and buffers allocated by
  bus_dmamem_alloc() (in both situations we known they're allocated
  on cacheline boundaries and don't need bouncing).
- Various minor reformatting and cleanups.