netlink: specs: rt-route: specify fixed-header at operations level

netlink: specs: rt-route: specify fixed-header at operations level

The C codegen currently stores the fixed-header as part of family
info, so it only supports one fixed-header type per spec. Luckily
all rtm route message have the same fixed header so just move it up
to the higher level.

Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20250410014658.782120-3-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>