net/mlx5: fix Rx/Tx stats concurrency

net/mlx5: fix Rx/Tx stats concurrency

Queue statistics are being continuously updated in Rx/Tx burst
routines while handling traffic. In addition to that, statistics
can be reset (written with zeroes) on statistics reset in other
threads, causing a race condition, which in turn could result in
wrong stats.

The patch provides an approach with reference values, allowing
the actual counters to be writable within Rx/Tx burst threads
only, and updating reference values on stats reset.

Fixes: 87011737b715 ("mlx5: add software counters")
Cc: stable@dpdk.org
Signed-off-by: Raja Zidane <rzidane@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>