net/mlx5: fix last completed built descriptor

net/mlx5: fix last completed built descriptor

The routine sending packets with Multi-Packet Write method assigns
the wqe_last variable with transmit descriptor (WQE - work queue entry)
being built. If send queue is close to full state, the WQE has no data
yet (trying to put the first packet) and there is no enough space
in descriptor for the next packet the WQE is discarded and the stored
wqe_last value becomes invalid - points to the discarded WQE.

The mlx5_tx_burst_request_completion() routine might set the completion
request flags in the WQE pointed by wqe_last, it is safe, but the next
mlx5_tx_burst call uses the WQE as the first free one and request
completion flags might be overwritten and completion request will be
lost causing the transmit  datapath malfunction.

Fixes: 8b581c690a54 ("net/mlx5: move Tx complete request routine")
Cc: stable@dpdk.org
Signed-off-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>
Acked-by: Matan Azrad <matan@mellanox.com>