ethtool: use the rss context XArray in ring deactivation safety-check

ethtool_get_max_rxfh_channel() gets called when user requests
deactivating Rx channels. Check the additional RSS contexts, too.

While we do track whether RSS context has an indirection
table explicitly set by the user, no driver looks at that bit.
Assume drivers won't auto-regenerate the additional tables,
to be safe.

Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/20240710174043.754664-3-kuba@kernel.org


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