virtio: order used ring after used index read

On SMP guests, reads from the ring might bypass used index reads. This causes guest crashes because host writes to used index to signal ring data readiness.
Fix this by inserting rmb before used ring reads.

---

Rusty, I see guest crashes with virtio net on vhost sometimes, and the following seems to help.
This seems an obviously correct thing to do, anyway. If you think so, too, can this be queued for 2.6.32 please? Thanks,

 drivers/virtio/virtio_ring.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)

index f536005..4c4df3e 100644
@@ -285,6 +285,9 @@ static void *vring_get_buf(struct virtqueue *_vq, unsigned int *len)
 		return NULL;
 	}
 
+        /* Only get used array entries after they have been exposed by host. */
+	rmb();
+
 	i = vq->vring.used->ring[vq->last_used_idx%vq->vring.num].id;
 	*len = vq->vring.used->ring[vq->last_used_idx%vq->vring.num].len;
 

1.6.5.rc2