add a new dma_alloc_noncontiguous API

Subject: add a new dma_alloc_noncontiguous API
From: Christoph Hellwig <hch@xxxxxx>
Date: Thu, 28 Jan 2021 15:58:31 +0100
Cc: Robin Murphy <robin.murphy@xxxxxxx>, linux-doc@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, linux-media@xxxxxxxxxxxxxxx

Hi all,

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

Follow-Ups:
- [PATCH 4/6] dma-iommu: refactor iommu_dma_alloc_remap
  - From: Christoph Hellwig
- [PATCH 6/6] media: uvcvideo: Use dma_alloc_noncontiguos API
  - From: Christoph Hellwig
- [PATCH 5/6] dma-iommu: implement ->alloc_noncontiguous
  - From: Christoph Hellwig
- [PATCH 2/6] dma-mapping: add a dma_mmap_pages helper
  - From: Christoph Hellwig
- [PATCH 3/6] dma-mapping: add a dma_alloc_noncontiguous API
  - From: Christoph Hellwig
- [PATCH 1/6] dma-mapping: remove the {alloc,free}_noncoherent methods
  - From: Christoph Hellwig