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>