selftests: drv-net: rss_ctx: test flow rehashing without impacting traffic

Some workloads may want to rehash the flows in response to an imbalance.
Most effective way to do that is changing the RSS key. Check that changing
the key does not cause link flaps or traffic disruption.

Disrupting traffic for key update is not incorrect, but makes the key
update unusable for rehashing under load.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20240708213627.226025-6-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>