MFC r256637:

  When calculating the number of bounce pages needed, round the maxsize
  up to a multiple of PAGE_SIZE, and add one page because there can always
  be one more boundary crossing than the number of pages in the transfer.
MFC r256638:

  Add cases for the combinations of busdma sync op flags that we handle
  correctly by doing nothing, then add a panic for the default case, because
  that implies that some driver asked for a sync (probably incorrectly) and
  nothing was done.