veth: Implement VLAN tag XDP hint

In order to test VLAN tag hint in hardware-independent selftests, implement
newly added hint in veth driver.

Acked-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Larysa Zaremba <larysa.zaremba@intel.com>
Link: https://lore.kernel.org/r/20231205210847.28460-13-larysa.zaremba@intel.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>