virtio_net: fix possible dim status unrecoverable

When the dim worker is scheduled, if it no longer needs to issue
commands, dim may not be able to return to the working state later.

For example, the following single queue scenario:
  1. The dim worker of rxq0 is scheduled, and the dim status is
     changed to DIM_APPLY_NEW_PROFILE;
  2. dim is disabled or parameters have not been modified;
  3. virtnet_rx_dim_work exits directly;

Then, even if net_dim is invoked again, it cannot work because the
state is not restored to DIM_START_MEASURE.

Fixes: 6208799 ("virtio-net: support rx netdim")
Signed-off-by: Heng Qi <[email protected]>
Reviewed-by: Jiri Pirko <[email protected]>
Acked-by: Michael S. Tsirkin <[email protected]>
Reviewed-by: Xuan Zhuo <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: Heng Qi

drivers/net/virtio_net.c

@@ -4419,9 +4419,9 @@ static void virtnet_rx_dim_work(struct work_struct *work)
 		if (err)
 			pr_debug("%s: Failed to send dim parameters on rxq%d\n",
 				 dev->name, qnum);
-		dim->state = DIM_START_MEASURE;
 	}
 out:
+	dim->state = DIM_START_MEASURE;
 	mutex_unlock(&rq->dim_lock);
 }