net/mlx5: fix mbuf freeing in vectorized MPRQ

net/mlx5: fix mbuf freeing in vectorized MPRQ

Wrong index is used to find mbufs belonging to an application in
the rxq_free_elts_sprq() function in the case of vectorized MPRQ.
elts_ci points to the last allocated mbuf in this case, not rq_ci.
Use this field to avoid double free of mbuf and segmentation fault.

Fixes: 0f20acbf5eda ("net/mlx5: implement vectorized MPRQ burst")
Cc: stable@dpdk.org
Signed-off-by: Alexander Kozyrev <akozyrev@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>