As pointed out by Jakub Kicinski, we ethtool netlink code should respond
with an error if request head has flags set which are not recognized by[...]
kernel, either as a mistake or because it expects functionality introduced
in later kernel versions.
To avoid unnecessary roundtrips, use extack cookie to provide the
information about supported request flags.
Signed-off-by: Michal Kubecek <mkubecek@xxxxxxx>