ethtool: prevent endless loop if eeprom size is smaller than announced

It shouldn't happen, but can happen that readable eeprom size is smaller
than announced. Then we would be stuck in an endless loop here because
after reaching the actual end reads return eeprom.len = 0. I faced this
issue when making a mistake in driver development. Detect this scenario
and return an error.
Signed-off-by: NHeiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ethtool/ioctl.c

@@ -1537,6 +1537,10 @@ static int ethtool_get_any_eeprom(struct net_device *dev, void __user *useraddr,
 		ret = getter(dev, &eeprom, data);
 		if (ret)
 			break;
+		if (!eeprom.len) {
+			ret = -EIO;
+			break;
+		}
 		if (copy_to_user(userbuf, data, eeprom.len)) {
 			ret = -EFAULT;
 			break;