RDMA/mlx5: Implement the query ucontext functionality

Implement the query ucontext functionality by returning the original
ucontext data as part of an extra mlx5 attribute that holds the driver
UAPI response.

Link: https://lore.kernel.org/r/20200630093916.332097-6-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>