net/mlx5: fix updating total length of multi-packet send

net/mlx5: fix updating total length of multi-packet send

The total length field in descriptor of inlined multi-packet send must be
updated before closing a session. There's possibility of updating it
afterward. This bug might cause one packet out of MLX5_MPW_DSEG_MAX gets
silently dropped by HW and impact performance, especially lossless test.

Fixes: 230189d9ff22 ("net/mlx5: support multi-packet send")
Cc: stable@dpdk.org
Signed-off-by: Yongseok Koh <yskoh@mellanox.com>
Acked-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>