- Since the coherent DMA allocator is not currently compatible with
  HUGETLB, ensure that we fail at compile time should this option be
  selected.
- Move struct device/dma mask handling into consistent_alloc().
- Complain if someone tries to allocate coherent memory larger than
  their DMA mask will permit.
- Rename consistent_alloc() to __dma_alloc() to ensure that there
  is no confusion that consistent_alloc() has gone.
- Ensure warnings and errors report the correct function name.