lguest: Make sure interrupt is allocated ok by lguest_setup_irq

Make sure the interrupt is allocated correctly by lguest_setup_irq (check the
return value of irq_alloc_desc_at for -ENOMEM)

Signed-off-by: Stratos Psomadakis <psomas@cslab.ece.ntua.gr>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> (cleanups and commentry)

drivers/lguest/lguest_device.c

@@ -241,7 +241,7 @@ static void lg_notify(struct virtqueue *vq)
 }
 
 /* An extern declaration inside a C file is bad form.  Don't do it. */
-extern void lguest_setup_irq(unsigned int irq);
+extern int lguest_setup_irq(unsigned int irq);
 
 /*
  * This routine finds the Nth virtqueue described in the configuration of
@@ -304,7 +304,9 @@ static struct virtqueue *lg_find_vq(struct virtio_device *vdev,
 	}
 
 	/* Make sure the interrupt is allocated. */
-	lguest_setup_irq(lvq->config.irq);
+	err = lguest_setup_irq(lvq->config.irq);
+	if (err)
+		goto destroy_vring;
 
 	/*
 	 * Tell the interrupt for this virtqueue to go to the virtio_ring
@@ -317,7 +319,7 @@ static struct virtqueue *lg_find_vq(struct virtio_device *vdev,
 	err = request_irq(lvq->config.irq, vring_interrupt, IRQF_SHARED,
 			  dev_name(&vdev->dev), vq);
 	if (err)
-		goto destroy_vring;
+		goto free_desc;
 
 	/*
 	 * Last of all we hook up our 'struct lguest_vq_info" to the
@@ -326,6 +328,8 @@ static struct virtqueue *lg_find_vq(struct virtio_device *vdev,
 	vq->priv = lvq;
 	return vq;
 
+free_desc:
+	irq_free_desc(lvq->config.irq);
 destroy_vring:
 	vring_del_virtqueue(vq);
 unmap: