vdpa/mlx5: Fix needs_teardown flag calculation

needs_teardown is a device flag that indicates when virtual queues need
to be recreated. This happens for certain configuration changes: queue
size and some specific features.

Currently, the needs_teardown state can be incorrectly reset by
subsequent .set_vq_num() calls. For example, for 1 rx VQ with size 512
and 1 tx VQ with size 256:

.set_vq_num(0, 512) -> sets needs_teardown to true (rx queue has a
                       non-default size)
.set_vq_num(1, 256) -> sets needs_teardown to false (tx queue has a
                       default size)

This change takes into account the previous value of the needs_teardown
flag when re-calculating it during VQ size configuration.

Fixes: 0fe963d ("vdpa/mlx5: Re-create HW VQs under certain conditions")
Signed-off-by: Dragos Tatulea <dtatulea@nvidia.com>
Reviewed-by: Shahar Shitrit <shshitrit@nvidia.com>
Reviewed-by: Si-Wei Liu <si-wei.liu@oracle.com>
Tested-by: Si-Wei Liu<si-wei.liu@oracle.com>
Message-Id: <20250604184802.2625300-1-dtatulea@nvidia.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>

Author: dtatulea

drivers/vdpa/mlx5/net/mlx5_vnet.c

@@ -2491,7 +2491,7 @@ static void mlx5_vdpa_set_vq_num(struct vdpa_device *vdev, u16 idx, u32 num)
         }
 
 	mvq = &ndev->vqs[idx];
-	ndev->needs_teardown = num != mvq->num_ent;
+	ndev->needs_teardown |= num != mvq->num_ent;
 	mvq->num_ent = num;
 }