ethtool: rss: support setting hfunc via Netlink

ethtool: rss: support setting hfunc via Netlink

Support setting RSS hash function / algo via ethtool Netlink.
Like IOCTL we don't validate that the function is within the
range known to the kernel. The drivers do a pretty good job
validating the inputs, and the IDs are technically "dynamically
queried" rather than part of uAPI.

Only change should be that in Netlink we don't support user
explicitly passing ETH_RSS_HASH_NO_CHANGE (0), if no change
is requested the attribute should be absent.

The ETH_RSS_HASH_NO_CHANGE is retained in driver-facing
API for consistency (not that I see a strong reason for it).

Reviewed-by: Gal Pressman <gal@nvidia.com>
Link: https://patch.msgid.link/20250716000331.1378807-6-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>