vhost: fix async packed ring batch datapath

vhost: fix async packed ring batch datapath

We assume that in the sync path, if there is no buffer wrap in the
avail descriptors fetched in a batch, there is no buffer wrap in the
used descriptors which need to be written back in this batch, but
this assumption is wrong in the async path since there are inflight
descriptors which are processed by the DMA device.

This patch refactors the batch copy code and adds used ring buffer
wrap check as a batch copy condition to fix this issue.

Fixes: 873e8dad6f49 ("vhost: support packed ring in async datapath")
Cc: stable@dpdk.org
Signed-off-by: Cheng Jiang <cheng1.jiang@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>