vhost: fix packed ring descriptor update in async enqueue

vhost: fix packed ring descriptor update in async enqueue

If the packet uses multiple descriptors and its descriptor indices are
wrapped, the first descriptor flag is not updated last, which may cause
virtio read the incomplete packet. For example, given a packet uses 64
descriptors, and virtio ring size is 256, and its descriptor indices are
224~255 and 0~31, current implementation will update 224~255 descriptor
flags earlier than 0~31 descriptor flags.

This patch fixes this issue by updating descriptor flags in one loop,
so that the first descriptor flag is always updated last.

Fixes: 873e8dad6f49 ("vhost: support packed ring in async datapath")
Signed-off-by: Jiayu Hu <jiayu.hu@intel.com>
Reviewed-by: Chenbo Xia <chenbo.xia@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>