netlink: specs: rt_link: decode ip6tnl, vti and vti6 link attrs

Some of our tests load vti and ip6tnl so not being able to decode
the link attrs gets in the way of using Python YNL for testing.

Decode link attributes for ip6tnl, vti and vti6.

ip6tnl uses IFLA_IPTUN_FLAGS as u32, while ipv4 and sit expect
a u16 attribute, so we have a (first?) subset type override...

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250107022820.2087101-4-kuba@kernel.org


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