RDMA/mlx5: Fix definition of mlx5_ib_create_qp_resp

This structure is pushed down the ex and the non-ex path, so it needs to be
aligned to 8 bytes to go through ex without implicit padding.

Old user space will provide 4 bytes of resp on !ex and 8 bytes on ex, so
take the approach of just copying the minimum length.

New user space will consistently provide 8 bytes in both cases.

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