net/mlx5: reduce Tx completion index memory loads

net/mlx5: reduce Tx completion index memory loads

There is a non-optimal check if doorbell is needed present in the
mlx5_tx_handle_completion() function. Advancing a copy of the txq
consumer index and checking this copy with initial value causes
unnecessary memory loads and hurts the performance. It is better to
have a simple small boolean variable for this purpose. That allows
to eliminate all the excessive memory operations with the txq consumer
index and restore the performance of the tx completions.

Fixes: 1fd9af05e44e ("net/mlx5: update Tx error handling routine")
Cc: stable@dpdk.org
Signed-off-by: Alexander Kozyrev <akozyrev@mellanox.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>