ethtool: rss: factor out populating response from context

ethtool: rss: factor out populating response from context

Similarly to previous change, factor out populating the response.
We will use this after the context was allocated to send a notification
so this time factor out from the additional context handling, rather
than context 0 handling (for request context didn't exist, for response
it does).

Reviewed-by: Gal Pressman <gal@nvidia.com>
Link: https://patch.msgid.link/20250717234343.2328602-5-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>