tuntap: fix error return code in tun_set_iff()

Fix to return a negative error code from the error handling
case instead of 0, as returned elsewhere in this function.

[ Bug added in linux-3.8 , commit 4008e97
  ("tuntap: fix ambigious multiqueue API") ]

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -1594,7 +1594,7 @@ static int tun_set_iff(struct net *net, struct file *file, struct ifreq *ifr)
 
 		if (tun->flags & TUN_TAP_MQ &&
 		    (tun->numqueues + tun->numdisabled > 1))
-			return err;
+			return -EBUSY;
 	}
 	else {
 		char *name;