RDMA/uverbs: Track dmabuf memory regions

The dmabuf memory registrations are missing the restrack handling and
hence do not appear in rdma tool.

Fixes: bfe0cc6e ("RDMA/uverbs: Add uverbs command for dma-buf based MR registration")
Link: https://lore.kernel.org/r/20210812135607.6228-1-galpress@amazon.com


Signed-off-by: Gal Pressman <galpress@amazon.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>