When upstream_bridge_distance() is called, store the method required to map
the DMA transfers in an xarray so it can be looked up efficiently on the
hot path in pci_p2pdma_map_sg().

Link: https://lore.kernel.org/r/20190730163545.4915-12-logang@deltatee.com
Link: https://lore.kernel.org/r/20190812173048.9186-12-logang@deltatee.comSigned-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>