For mlx5 if ATS is enabled in the PCI config then the device will use ATS
requests for only certain DMA operations. This has to be opted in by the
SW side based on the mkey or umem settings.

ATS slows down the PCI performance, so it should only be set in cases when
it is needed. All of these cases revolve around optimizing PCI P2P
transfers and avoiding bad cases where the bus just doesn't work.

Link: https://lore.kernel.org/r/4-v1-bd147097458e+ede-umem_dmabuf_jgg@nvidia.comSigned-off-by: Jason Gunthorpe <jgg@nvidia.com>