net: ethtool: rss: add notifications

In preparation for RSS_SET handling in ethnl introduce Netlink
notifications for RSS. Only cover modifications, not creation
and not removal of a context, because the latter may deserve
a different notification type. We should cross that bridge
when we add the support for context add / remove via Netlink.

Link: https://patch.msgid.link/20250623231720.3124717-7-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/netlink/specs/ethtool.yaml

@@ -2492,6 +2492,13 @@ operations:
         attributes:
           - header
           - events
+    -
+      name: rss-ntf
+      doc: |
+        Notification for change in RSS configuration.
+        For additional contexts only modifications are modified, not creation
+        or removal of the contexts.
+      notify: rss-get
 
 mcast-groups:
   list: