shadow_used_ring will be introduced later. Since then last avail
idx will not be updated together with last used idx.
So, here we use last_avail_idx for avail ring reservation.
Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Signed-off-by: Zhihong Wang <zhihong.wang@intel.com>
Reviewed-by: Jianbo Liu <jianbo.liu@linaro.org>
Tested-by: Maxime Coquelin <maxime.coquelin@redhat.com>