net/mlx5: fix Rx queue resource cleanup

net/mlx5: fix Rx queue resource cleanup

mlx5_rxq_start() allocates rxq_ctrl->obj and frees it on failure,
but did not set it to NULL. Later mlx5_rxq_release() could not recognize
this object is already freed and attempted to release its resources,
resulting in a crash:

    Configuring Port 0 (socket 0)
    mlx5_common: Failed to create RQ using DevX
    mlx5_common: Can't create DevX RQ object.
    mlx5_net: Port 0 Rx queue 0 RQ creation failure.
    Segmentation fault

Set rxq_ctrl->obj to NULL after it is freed to skip resource release.

Fixes: 1260a87b2889 ("net/mlx5: share Rx control code")
Cc: stable@dpdk.org
Signed-off-by: Dmitry Kozlyuk <dkozlyuk@nvidia.com>
Acked-by: Matan Azrad <matan@nvidia.com>