selftests: forwarding: hw_stats_l3_gre: Skip when using veth pairs

Layer 3 hardware stats cannot be used when the underlying interfaces are
veth pairs, resulting in failures:

# ./hw_stats_l3_gre.sh
TEST: ping gre flat [ OK ]
TEST: Test rx packets: [FAIL]
Traffic not reflected in the counter: 0 -> 0
TEST: Test tx packets: [FAIL]
Traffic not reflected in the counter: 0 -> 0

Fix by skipping the test when used with veth pairs.

Fixes: 813f97a26860 ("selftests: forwarding: Add a tunnel-based test for L3 HW stats")
Reported-by: Mirsad Todorovac <mirsad.todorovac@alu.unizg.hr>
Closes: https://lore.kernel.org/netdev/adc5e40d-d040-a65e-eb26-edf47dac5b02@alu.unizg.hr/
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Tested-by: Mirsad Todorovac <mirsad.todorovac@alu.unizg.hr>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://lore.kernel.org/r/20230808141503.4060661-10-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

selftests: forwarding: Add a tunnel-based test for L3 HW stats

Add a selftest that uses an IPIP topology and tests that L3 HW stats
reflect the traffic in the tunnel.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>