RDMA/mlx5: Introduce UAPI to query PD attributes

Introduce UAPI to query PD attributes, this can be used to retrieve PD
attributes by having the PD handle of the created one and owning the
command FD for the ucontxet.

Link: https://lore.kernel.org/r/20200630093916.332097-7-leon@kernel.org


Signed-off-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>