net/mlx5: fix Tx descriptor with VLAN insertions

net/mlx5: fix Tx descriptor with VLAN insertions

If VLAN tag insertion transmit offload is engaged
(DEV_TX_OFFLOAD_VLAN_INSERT in tx queue configuration is set)
the transmit descriptor may be built with wrong format, due to
packet length is not adjusted. Also, the ring buffer wrap up
is not handled correctly.

Fixes: 18a1c20044c0 ("net/mlx5: implement Tx burst template")
Cc: stable@dpdk.org
Signed-off-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>
Acked-by: Matan Azrad <matan@mellanox.com>