selftests/net: test l2 tunnel TOS/TTL inheriting

selftests/net: test l2 tunnel TOS/TTL inheriting

There are currently 3 ip tunnels that are capable of carrying
L2 traffic: gretap, vxlan and geneve.
They all are capable to inherit the TOS/TTL for the outer
IP-header from the inner frame.

Add a test that verifies that these fields are correctly inherited.

These tests failed before the following commits:
66ced498d1fc ("ip6_tunnel: allow to inherit from VLAN encapsulated IP")
39c45ee5a03e ("ip6_gre: use actual protocol to select xmit")
2b3daf1e0c60 ("ip6_gre: set DSCP for non-IP")
fe3bca22e1c0 ("ip_tunnel: allow to inherit from VLAN encapsulated IP")
caf4eee21a37 ("ip_tunnels: allow VXLAN/GENEVE to inherit TOS/TTL from VLAN")
99470e99d939 ("geneve: do not use RT_TOS for IPv6 flowlabel")
289f831ac5ee ("geneve: fix TOS inheriting for ipv4")

Signed-off-by: Matthias May <matthias.may@westermo.com>
Link: https://lore.kernel.org/r/20220817073649.26117-1-matthias.may@westermo.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>