RDMA/umem: Support importing dma-buf as user memory region

Dma-buf is a standard cross-driver buffer sharing mechanism that can be
used to support peer-to-peer access from RDMA devices.

Device memory exported via dma-buf is associated with a file descriptor.
This is passed to the user space as a property associated with the buffer
allocation. When the buffer is registered as a memory region, the file
descriptor is passed to the RDMA driver along with other parameters.

Implement the common code for importing dma-buf object and mapping dma-buf
pages.

Link: https://lore.kernel.org/r/1608067636-98073-2-git-send-email-jianxin.xiong@intel.com


Signed-off-by: Jianxin Xiong <jianxin.xiong@intel.com>
Reviewed-by: Sean Hefty <sean.hefty@intel.com>
Acked-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Acked-by: Christian Koenig <christian.koenig@amd.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>