kernel - Adjust BUS_DMASYNC_* API

kernel - Adjust BUS_DMASYNC_* API

* Change the BUS_DMASYNC_* enumeration to a bitmask to allow multiple
  flags to be specified in one call.

* Now more compatible with FreeBSD and one less thing to worry about when
  porting a driver.

* While we are at it, allow add_bounce_page() to modify the size.  This has
  no effect on the current codebase since bounce pages are page-aligned (so the
  page-residual will never be less than the passed address), but is more
  correct.

* Fixes bounce-buffer issue with if_ath.