net-sysfs: restore behavior for not running devices

modprobe dummy dumdummies=1

Old behavior :

$ cat /sys/class/net/dummy0/carrier
cat: /sys/class/net/dummy0/carrier: Invalid argument

After blamed commit, an empty string is reported.

$ cat /sys/class/net/dummy0/carrier
$

In this commit, I restore the old behavior for carrier,
speed and duplex attributes.

Fixes: 79c61899b5ee ("net-sysfs: remove rtnl_trylock from device attributes")
Signed-off-by: Eric Dumazet <[e-mail]>
Reported-by: Marco Leogrande <[e-mail]>
Reviewed-by: Antoine Tenart <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/core/net-sysfs.c

···
 			    struct device_attribute *attr, char *buf)
 {
 	struct net_device *netdev = to_net_dev(dev);
-	int ret = -EINVAL;
+	int ret;
 
 	ret = sysfs_rtnl_lock(&dev->kobj, &attr->attr, netdev);
 	if (ret)
 		return ret;
 
+	ret = -EINVAL;
 	if (netif_running(netdev)) {
 		/* Synchronize carrier state with link watch,
 		 * see also rtnl_getlink().
···
 	if (ret)
 		return ret;
 
+	ret = -EINVAL;
 	if (netif_running(netdev)) {
 		struct ethtool_link_ksettings cmd;
 
···
 	if (ret)
 		return ret;
 
+	ret = -EINVAL;
 	if (netif_running(netdev)) {
 		struct ethtool_link_ksettings cmd;