selftests: forwarding: router_mpath_nh_lib: Don't skip, xfail on veth

When the NH group stats tests are currently run on a veth topology, the
HW-stats leg of each test is SKIP'ped. But kernel networking CI interprets
skips as a sign that tooling is missing, and prompts maintainer
investigation. Lack of capability to pass a test should be expressed as
XFAIL.

Selftests that require HW should normally be put in drivers/net/hw, but
doing so for the NH counter selftests would just lead to a lot of
duplicity.

So instead, introduce a helper, xfail_on_veth(), which can be used to mark
selftests that should XFAIL instead of FAILing when run on a veth topology.
On non-veth topology, they don't do anything.

Use the helper in the HW-stats part of router_mpath_nh_lib selftest.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Link: https://lore.kernel.org/r/15f0ab9637aa0497f164ec30e83c1c8f53d53719.1711464583.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>