net/mlx5: fix redundant free of Tx buffer

net/mlx5: fix redundant free of Tx buffer

SW completion ring of Tx (txq->elts) stores individual mbufs even if a
multi-segmented packet is sent. rte_pktmbuf_free_seg() must be used when
cleaning up the completion ring. Otherwise, chained mbufs are redundantly
freed and finally it would cause a crash.

Fixes: 1d88ba171942 ("net/mlx5: refactor Tx data path")
CC: stable@dpdk.org
Signed-off-by: Yongseok Koh <yskoh@mellanox.com>
Acked-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>