RDMA/uverbs: Add uverbs command for dma-buf based MR registration

Implement a new uverbs ioctl method for memory registration with file
descriptor as an extra parameter.

Link: https://lore.kernel.org/r/1608067636-98073-4-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>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>