vdpa/mlx5: Use write memory barrier after updating CQ index

vdpa/mlx5: Use write memory barrier after updating CQ index

Make sure to put dma write memory barrier after updating CQ consumer
index so the hardware knows that there are available CQE slots in the
queue.

Failure to do this can cause the update of the RX doorbell record to get
updated before the CQ consumer index resulting in CQ overrun.

Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for supported mlx5 devices")
Signed-off-by: Eli Cohen <elic@nvidia.com>
Link: https://lore.kernel.org/r/20201209140004.15892-1-elic@nvidia.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>