vhost: fix packed ring zero-copy

vhost: fix packed ring zero-copy

Available buffer ID should be stored in the zmbuf in the packed-ring
dequeue path. There's no guarantee that local queue avail index is
equal to buffer ID.

Fixes: d1eafb532268 ("vhost: add packed ring zcopy batch and single dequeue")
Cc: stable@dpdk.org
Signed-off-by: Marvin Liu <yong.liu@intel.com>
Reported-by: Yinan Wang <yinan.wang@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>