r343962:
  Properly handle alignment requests bigger that page size.
   - for now, alignments bigger that page size is allowed only for buffers
     allocated by bus_dmamem_alloc(), cover this fact by KASSERT.
   - never bounce buffers allocated by bus_dmamem_alloc(), these always
   comply
     with the required rules (alignment, boundary, address range).
r343965:
  Fix bug introduced by r343962.  DMAMAP_DMAMEM_ALLOC is property of dmamap,
  not dmatag.