vhost: fix corner case for enqueue operation

vhost: fix corner case for enqueue operation

When performing enqueue operations on the split and packed rings,
if the reserved buffer length from the descriptor table exceeds
65535, the returned length by fill_vec_buf_split/_packed()
overflows. This patch is to avoid this corner case.

Fixes: f689586bc060 ("vhost: shadow used ring update")
Fixes: fd68b4739d2c ("vhost: use buffer vectors in dequeue path")
Fixes: 2f3225a7d69b ("vhost: add vector filling support for packed ring")
Fixes: 37f5e79a271d ("vhost: add shadow used ring support for packed rings")
Fixes: a922401f35cc ("vhost: add Rx support for packed ring")
Fixes: ae999ce49dcb ("vhost: add Tx support for packed ring")
Cc: stable@dpdk.org
Signed-off-by: Jiayu Hu <jiayu.hu@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>