selftests: forwarding: Add a selftest for the new reserved_bits UAPI

Run VXLAN packets through a gateway. Flip individual bits of the packet
and/or reserved bits of the gateway, and check that the gateway treats the
packets as expected.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://patch.msgid.link/388bef3c30ebc887d4e64cd86a362e2df2f2d2e1.1733412063.git.petrm@nvidia.com


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