net/virtio: fix refill order in packed ring datapath

net/virtio: fix refill order in packed ring datapath

The front-end should refill the descriptor with the mbuf indicated by
the buff_id rather then the index of used descriptor. Back-end may
return buffers out of order if async copy mode is enabled.

When initializing rxq, refill the descriptors in order as buff_id is
not available at that time.

Fixes: a76290c8f1cf ("net/virtio: implement Rx path for packed queues")
Cc: stable@dpdk.org
Signed-off-by: Cheng Jiang <cheng1.jiang@intel.com>
Signed-off-by: Marvin Liu <yong.liu@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>