ethtool: rss: support setting hkey via Netlink

Support setting RSS hashing key via ethtool Netlink.
Use the Netlink policy to make sure user doesn't pass
an empty key, "resetting" the key is not a thing.

Reviewed-by: Gal Pressman <gal@nvidia.com>
Link: https://patch.msgid.link/20250716000331.1378807-7-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>