tun: allow positive return values on dev_get_valid_name() call

If the name argument of dev_get_valid_name() contains "%d", it will try
to assign it a unit number in __dev__alloc_name() and return either the
unit number (>= 0) or an error code (< 0).
Considering positive values as error values prevent tun device creations
relying this mechanism, therefor we should only consider negative values
as errors here.
Signed-off-by: NJulien Gomes <julien@arista.com>
Acked-by: NCong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -2028,7 +2028,7 @@ static int tun_set_iff(struct net *net, struct file *file, struct ifreq *ifr)
 		if (!dev)
 			return -ENOMEM;
 		err = dev_get_valid_name(net, dev, name);
-		if (err)
+		if (err < 0)
 			goto err_free_dev;
 
 		dev_net_set(dev, net);