net/failsafe: stat support enhancement

net/failsafe: stat support enhancement

The previous stats code returned only the current TX sub
device stats.

This enhancement extends it to return the sum of all sub
devices stats with history of removed sub-devices.

Dedicated stats accumulator saves the stat history of all
sub device remove events.

Each failsafe sub device contains the last stats asked by
the user and updates the accumulator in removal time.

I would like to implement ultimate snapshot on removal time.
The stats_get API needs to be changed to return error in the
case it is too late to retrieve statistics.
By this way, failsafe can get stats snapshot in removal interrupt
callback for each PMD which can give stats after removal event.

Signed-off-by: Matan Azrad <matan@mellanox.com>
Acked-by: Gaetan Rivet <gaetan.rivet@6wind.com>