netlink: specs: rt-route: remove the fixed members from attrs

netlink: specs: rt-route: remove the fixed members from attrs

The purpose of the attribute list is to list the attributes
which will be included in a given message to shrink the objects
for families with huge attr spaces. Fixed headers are always
present in their entirety (between netlink header and the attrs)
so there's no point in listing their members. Current C codegen
doesn't expect them and tries to look them up in the attribute space.

Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20250410014658.782120-5-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>