netlink: specs: rt-link: decode ip6gre

netlink: specs: rt-link: decode ip6gre

Driver tests now require GRE tunnels, while we don't configure
them with YNL, YNL will complain when it sees link types it
doesn't recognize. Teach it decoding ip6gre tunnels. The attrs
are largely the same as IPv4 GRE.

Correct the type of encap-limit, but note that this attr is
only used in ip6gre, so the mistake didn't matter until now.

Fixes: 0d0f4174f6c8 ("selftests: drv-net: add a simple TSO test")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20250603135357.502626-3-kuba@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>