net/failsafe: improve stats accuracy

net/failsafe: improve stats accuracy

The stats_get API was changed to signal a potential failure to read
stats. Furthermore, some PMDs are able to provide statistics even
after a removal event occurred.

Considering this, the fail-safe can try to access the latest
statistics of a PMD to improve statistics accuracy.

Attempt an ultimate statistics read on removal time; if that
fails, use the latest recorded snapshot.

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