virtio-ring: fix DMA metadata flags

The flags are currently overwritten, leading to the wrong direction
being passed to the DMA unmap functions.

Fixes: 72b5e895

 ("virtio-ring: store DMA metadata in desc_extra for split virtqueue")
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Link: https://lore.kernel.org/r/20211026133100.17541-1-vincent.whitchurch@axis.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>