virtio_ring: add in order support

This patch implements in order support for both split virtqueue and
packed virtqueue. Performance could be gained for the device where the
memory access could be expensive (e.g vhost-net or a real PCI device):

Benchmark with KVM guest:

Vhost-net on the host: (pktgen + XDP_DROP):

         in_order=off | in_order=on | +%
    TX:  4.51Mpps     | 5.30Mpps    | +17%
    RX:  3.47Mpps     | 3.61Mpps    | + 4%

Vhost-user(testpmd) on the host: (pktgen/XDP_DROP):

For split virtqueue:

         in_order=off | in_order=on | +%
    TX:  5.60Mpps     | 5.60Mpps    | +0.0%
    RX:  9.16Mpps     | 9.61Mpps    | +4.9%

For packed virtqueue:

         in_order=off | in_order=on | +%
    TX:  5.60Mpps     | 5.70Mpps    | +1.7%
    RX:  10.6Mpps     | 10.8Mpps    | +1.8%

Benchmark also shows no performance impact for in_order=off for queue
size with 256 and 1024.

Reviewed-by: Eugenio Pérez <eperezma@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <20251230064649.55597-20-jasowang@redhat.com>

Author: jasowang