virtqueue: fix invalidate wrong cache region

The current code fetches the vring index before invalidating the cache.
This can lead to using an invalid index.
Fetch the index before invalidation to ensure a correct index.
Signed-off-by: Joshua Lin <jlin@petaio.com>

lib/virtio/virtqueue.c

@@ -271,10 +271,10 @@ void *virtqueue_get_available_buffer(struct virtqueue *vq, uint16_t *avail_idx,
 
 	VQUEUE_BUSY(vq);
 
+	head_idx = vq->vq_available_idx++ & (vq->vq_nentries - 1);
+
 	/* Avail.ring is updated by master, invalidate it */
 	VRING_INVALIDATE(vq->vq_ring.avail->ring[head_idx]);
-
-	head_idx = vq->vq_available_idx++ & (vq->vq_nentries - 1);
 	*avail_idx = vq->vq_ring.avail->ring[head_idx];
 
 	/* Invalidate the desc entry written by master before accessing it */
@@ -453,10 +453,10 @@ uint32_t virtqueue_get_desc_size(struct virtqueue *vq)
 
 	VQUEUE_BUSY(vq);
 
+	head_idx = vq->vq_available_idx & (vq->vq_nentries - 1);
+
 	/* Avail.ring is updated by master, invalidate it */
 	VRING_INVALIDATE(vq->vq_ring.avail->ring[head_idx]);
-
-	head_idx = vq->vq_available_idx & (vq->vq_nentries - 1);
 	avail_idx = vq->vq_ring.avail->ring[head_idx];
 
 	/* Invalidate the desc entry written by master before accessing it */