ethtool: rss: support removing contexts via Netlink

Implement removing additional RSS contexts via Netlink.
Technically it'd be possible to shoehorn the delete operation
into ethnl_request_ops-compatible handler. The code ends
up longer than open coded version, and I think we'll need
a custom way of sending notifications at some stage (if we
allow tying the context lifetime to the netlink socket, in
the future).

Link: https://patch.msgid.link/20250717234343.2328602-8-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/netlink/specs/ethtool.yaml

@@ -2706,6 +2706,24 @@ operations:
       doc: |
         Notification for creation of an additional RSS context.
       notify: rss-create-act
+    -
+      name: rss-delete-act
+      doc: Delete an RSS context.
+      attribute-set: rss
+      do:
+        request:
+          attributes:
+            - header
+            - context
+    -
+      name: rss-delete-ntf
+      doc: |
+        Notification for deletion of an additional RSS context.
+      attribute-set: rss
+      event:
+        attributes:
+          - header
+          - context
 
 mcast-groups:
   list: