net/mlx5: fix sync when handling Tx completions

net/mlx5: fix sync when handling Tx completions

Function mlx5_tx_complete() reads completion entry information
from Tx queue.
For some processors not having strongly-ordered memory model,
there has to be a memory barrier between reading the entry index
and the entry fields, in order to guarantee data is valid.

Fixes: 54d3fe948dba ("net/mlx5: poll completion queue once per a call")
Cc: stable@dpdk.org
Signed-off-by: Dekel Peled <dekelp@mellanox.com>
Acked-by: Shahaf Shuler <shahafs@mellanox.com>