vhost: fix packed ring index wrapping

vhost: fix packed ring index wrapping

Unlike split ring, packed ring does not mandate the ring size
to be a power of 2. So we have to use a modulo operation when
wrapping ring index.

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