vhost: fix index overflow for packed ring in async vhost

vhost: fix index overflow for packed ring in async vhost

We introduced some new indexes in packed ring of async vhost. They
will eventually overflow and lead to errors if the ring size is not
a power of 2. This patch is to check and keep these indexes within a
reasonable range.

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