ethtool: tsinfo: don't pass ERR_PTR to genlmsg_cancel on prepare failure

The goto err label leads to:

	genlmsg_cancel(skb, ehdr);
	return ret;

If ethnl_tsinfo_prepare_dump() failed, it has not started a genlmsg.
There's nothing to cancel, and passing an error pointer to
genlmsg_cancel() would cause a crash.

Fixes: b9e3f7dc9e ("net: ethtool: tsinfo: Enhance tsinfo to support several hwtstamp by net topology")
Reviewed-by: Maxime Chevallier <[email protected]>
Reviewed-by: Kory Maincent <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

net/ethtool/tsinfo.c

@@ -407,10 +407,8 @@ static int ethnl_tsinfo_dump_one_netdev(struct sk_buff *skb,
 			continue;
 
 		ehdr = ethnl_tsinfo_prepare_dump(skb, dev, reply_data, cb);
-		if (IS_ERR(ehdr)) {
-			ret = PTR_ERR(ehdr);
-			goto err;
-		}
+		if (IS_ERR(ehdr))
+			return PTR_ERR(ehdr);
 
 		reply_data->ts_info.phc_qualifier = ctx->pos_phcqualifier;
 		ret = ops->get_ts_info(dev, &reply_data->ts_info);