netlink: specs: rt-link: adjust AF_ nest for C codegen

The AF nest is indexed by AF ID, so it's a bit strange,
but with minor adjustments C codegen deals with it just fine.
Entirely unclear why the names have been in quotes here.

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20250418021706.1967583-7-kuba@kernel.org


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