MFC r269206, r269207, r269208, r269209, r269210, r269211, r269212, r269213,
    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.