virtio_ring: Check used descriptors are chain heads

When the device returns used buffers, it should refer to the descriptor
that is the head of the descriptor chain for that buffer. Confirm this
to be the case by tracking the head of descriptor chains that have been
made available to the device.

Signed-off-by: Andrew Scull <ascull@google.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

include/virtio_ring.h

@@ -61,6 +61,8 @@ struct vring_desc_shadow {
 	u32 len;
 	u16 flags;
 	u16 next;
+	/* Metadata about the descriptor. */
+	bool chain_head;
 };
 
 struct vring_avail {