net/mlx5: fix Tx metadata for multi-segment packet

Original patch implemented the use of match_metadata offload in the
different burst functions.
The concurrent use of match_metadata and multi_segs offloads was
not handled.

This patch updates function txq_scatter_v(), to pass metadata value
from mbuf to wqe, when indicated by offload flags.

Fixes: 6bd7fbd03c62 ("net/mlx5: support metadata as flow rule criteria")
Cc: stable@dpdk.org
Signed-off-by: Dekel Peled <dekelp@mellanox.com>
Acked-by: Yongseok Koh <yskoh@mellanox.com>