RDMA/mlx5: Reuse existing fields in parent QP storage object

Remove duplication of mlx5_ib_qp and mlx5_ib_gsi_qp fields.  This change
returns the memory footprint of mlx5_ib QP to be as it was before
embedding GSI QP.

Link: https://lore.kernel.org/r/20200926102450.2966017-3-leon@kernel.org
Reviewed-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>