ethtool: reject unrecognized request flags

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@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>