IB/mlx5: Fix stack info leak in mlx5_ib_alloc_ucontext()

We don't set "resp.reserved".  Since it's at the end of the struct
that means we don't have to copy it to the user.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Eli Cohen <eli@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>