ethtool: rss: support removing contexts via Netlink

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>