net: ethtool: use the tracking array for get_rxfh on custom RSS contexts

On 'ethtool -x' with rss_context != 0, instead of calling the driver to
 read the RSS settings for the context, just get the settings from the
 rss_ctx xarray, and return them to the user with no driver involvement.

Signed-off-by: Edward Cree <ecree.xilinx@gmail.com>
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Link: https://patch.msgid.link/2d0190fa29638f307ea720f882ebd41f6f867694.1719502240.git.ecree.xilinx@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>