Merge tag 'dma-mapping-4.17-2' of git://git.infradead.org/users/hch/dma-mapping

Pull dma-mapping fix from Christoph Hellwig:
 "Fix for one swiotlb regression in 2.16 from Takashi"

* tag 'dma-mapping-4.17-2' of git://git.infradead.org/users/hch/dma-mapping:
  swiotlb: fix unexpected swiotlb_alloc_coherent failures