ethtool: report missing header via ext_ack in the default handler

The actual presence check for the header is in
ethnl_parse_header_dev_get() but it's a few layers in,
and already has a ton of arguments so let's just pick
the low hanging fruit and check for missing header in
the default request handler.
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/ethtool/netlink.c

@@ -361,6 +361,9 @@ static int ethnl_default_doit(struct sk_buff *skb, struct genl_info *info)
 	ops = ethnl_default_requests[cmd];
 	if (WARN_ONCE(!ops, "cmd %u has no ethnl_request_ops\n", cmd))
 		return -EOPNOTSUPP;
+	if (GENL_REQ_ATTR_CHECK(info, ops->hdr_attr))
+		return -EINVAL;
+
 	req_info = kzalloc(ops->req_info_size, GFP_KERNEL);
 	if (!req_info)
 		return -ENOMEM;