um: vector: fix missing unlock on error in vector_net_open()

Add the missing unlock before return from function vector_net_open()
in the error handling case.

Fixes: ad1f62ab2bd4 ("High Performance UML Vector Network Driver")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/drivers/vector_kern.c

@@ -1156,8 +1156,10 @@ static int vector_net_open(struct net_device *dev)
 	struct vector_device *vdevice;
 
 	spin_lock_irqsave(&vp->lock, flags);
-	if (vp->opened)
+	if (vp->opened) {
+		spin_unlock_irqrestore(&vp->lock, flags);
 		return -ENXIO;
+	}
 	vp->opened = true;
 	spin_unlock_irqrestore(&vp->lock, flags);