RDMA/mlx5: Embed GSI QP into general mlx5_ib QP

The GSI QPs have different create flow from the regular QPs, but it is not
really needed. Update the code to use mlx5_ib_qp as a storage class for
all outside of GSI calls.

Link: https://lore.kernel.org/r/20200926102450.2966017-2-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>