RDMA/mlx5: Assign dev to DM MR

Currently, DM MR registration flow doesn't set the mlx5_ib_dev pointer and
can cause a NULL pointer dereference if userspace dumps the MR via rdma
tool.

Assign the IB device together with the other fields and remove the
redundant reference of mlx5_ib_dev from mlx5_ib_mr.

Cc: stable@vger.kernel.org
Fixes: 6c29f57e ("IB/mlx5: Device memory mr registration support")
Link: https://lore.kernel.org/r/20201203190807.127189-1-leon@kernel.org


Signed-off-by: Maor Gottlieb <maorg@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>