iommu/dma: support PCI P2PDMA pages in dma-iommu map_sg

Call pci_p2pdma_map_segment() when a PCI P2PDMA page is seen so the bus
address is set in the dma address and the segment is marked with
sg_dma_mark_bus_address(). iommu_map_sg() will then skip these segments.
Then, in __finalise_sg(), copy the dma address from the input segment
to the output segment. __invalidate_sg() must also learn to skip these
segments.

A P2PDMA page may have three possible outcomes when being mapped:
  1) If the data path between the two devices doesn't go through
     the root port, then it should be mapped with a PCI bus address
  2) If the data path goes through the host bridge, it should be mapped
     normally with an IOMMU IOVA.
  3) It is not possible for the two devices to communicate and thus
     the mapping operation should fail (and it will return -EREMOTEIO).

Similar to dma-direct, the sg_dma_mark_pci_p2pdma() flag is used to
indicate bus address segments. On unmap, P2PDMA segments are skipped
over when determining the start and end IOVA addresses.

With this change, the flags variable in the dma_map_ops is set to
DMA_F_PCI_P2PDMA_SUPPORTED to indicate support for P2PDMA pages.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>