tun: correctly report an error in tun_flow_init()

On error, the error code from tun_flow_init() is lost inside
tun_set_iff(), this patch fixes this by assigning the tun_flow_init()
error code to the "err" variable which is returned by
the tun_flow_init() function on error.
Signed-off-by: Paul Moore <pmoore@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -1591,7 +1591,8 @@ static int tun_set_iff(struct net *net, struct file *file, struct ifreq *ifr)
 
 		tun_net_init(dev);
 
-		if (tun_flow_init(tun))
+		err = tun_flow_init(tun);
+		if (err < 0)
 			goto err_free_dev;
 
 		dev->hw_features = NETIF_F_SG | NETIF_F_FRAGLIST |