dma-mapping: merge dma_unmap_page_attrs and dma_unmap_single_attrs

The two functions are exactly the same, so don't bother implementing
them twice.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Tested-by: Jesper Dangaard Brouer <brouer@redhat.com>
Tested-by: Tony Luck <tony.luck@intel.com>