selftests: forwarding: hw_stats_l3: Detect failure to install counters

Running this test makes little sense if the enabled l3_stats are not
actually reported as "used". This can signify a failure of a driver to
install the necessary counters, or simply lack of support for enabling
in-HW counters on a given netdevice. It is generally impossible to tell
from the outside which it is. But more likely than not, if somebody is
running this on veth pairs, they do not intend to actually test that a
certain piece of HW can install in-HW counters for the veth. It is more
likely they are e.g. running the test by mistake.

Therefore detect that the counter has not been actually installed. In that
case, if the netdevice is one end of a veth pair, SKIP. Otherwise FAIL.

Suggested-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Danielle Ratson <danieller@nvidia.com>
Tested-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://lore.kernel.org/r/a86817961903cca5cb0aebf2b2a06294b8aa7dea.1680704172.git.petrm@nvidia.com


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