virtio: correct the memory barrier in virtqueue_kick_prepare()

virtio: correct the memory barrier in virtqueue_kick_prepare()

Use virtio_mb() to make sure the available index to be exposed before
checking the the avail event. Otherwise we may get stale value of
avail event in guest and never kick the host after.

Note: this fixes a bug introduced by 91c29160a2c6187ce938c9f235f1c2dd06e519e0.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: stable@kernel.org