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>

Documentation/netlink/specs/rt-route.yaml

@@ -245,12 +245,12 @@ attribute-sets:
 
 operations:
   enum-model: directional
+  fixed-header: rtmsg
   list:
     -
       name: getroute
       doc: Dump route information.
       attribute-set: route-attrs
-      fixed-header: rtmsg
       do:
         request:
           value: 26
@@ -320,7 +320,6 @@ operations:
       name: newroute
       doc: Create a new route
       attribute-set: route-attrs
-      fixed-header: rtmsg
       do:
         request:
           value: 24
@@ -329,7 +328,6 @@ operations:
       name: delroute
       doc: Delete an existing route
       attribute-set: route-attrs
-      fixed-header: rtmsg
       do:
         request:
           value: 25