selftests: router_scale: Do not count failed routes

To check how many routes are installed in hardware, the test runs "ip
route" and greps for "offload", which includes routes with state
"offload_failed".

Till now, this wrong check was not found because after one failure in
route insertion, the driver moved to "abort" mode, which means that user
cannot try to add more routes.

The previous patch removed the abort mechanism and now failed routes are
counted as offloaded.

Fix this by not considering routes with "offload_failed" flag as
offloaded.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>