add a new dma_alloc_noncontiguous API v2

Hi all,

this series adds the new noncontiguous DMA allocation API requested by
various media driver maintainers.

Changes since v1:
 - document that flush_kernel_vmap_range and invalidate_kernel_vmap_range
   must be called once an allocation is mapped into KVA
 - add dma-debug support
 - remove the separate dma_handle argument, and instead create fully formed
   DMA mapped scatterlists
 - use a directional allocation in uvcvideo
 - call invalidate_kernel_vmap_range from uvcvideo