tuntap: correct the return value in tun_set_iff()

commit (3be8fbab tuntap: fix error return code in tun_set_iff()) breaks the
creation of multiqueue tuntap since it forbids to create more than one queues
for a multiqueue tuntap device. We need return 0 instead -EBUSY here since we
don't want to re-initialize the device when one or more queues has been already
attached. Add a comment and correct the return value to zero.

Reported-by: Jerry Chu <hkchu@google.com>
Cc: Jerry Chu <hkchu@google.com>
Cc: Wei Yongjun <weiyj.lk@gmail.com>
Cc: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Acked-by:  Jerry Chu <hkchu@google.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -1593,8 +1593,12 @@ static int tun_set_iff(struct net *net, struct file *file, struct ifreq *ifr)
 			return err;
 
 		if (tun->flags & TUN_TAP_MQ &&
-		    (tun->numqueues + tun->numdisabled > 1))
-			return -EBUSY;
+		    (tun->numqueues + tun->numdisabled > 1)) {
+			/* One or more queue has already been attached, no need
+			 * to initialize the device again.
+			 */
+			return 0;
+		}
 	}
 	else {
 		char *name;