selftests: drv-net: rss_ctx: check for all-zero keys

We had a handful of bugs relating to key being either all 0
or just reported incorrectly as all 0. Check for this in
the tests.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>