drop_monitor: fix trace_napi_poll_hit()

The net_dev of backlog napi is NULL, like below:

__get_cpu_var(softnet_data).backlog.dev == NULL

So, we should check it in napi tracepoint's probe function
Acked-by: NNeil Horman <nhorman@tuxdriver.com>
Signed-off-by: NXiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/drop_monitor.c

@@ -182,7 +182,8 @@ static void trace_napi_poll_hit(struct napi_struct *napi)
 	/*
 	 * Ratelimit our check time to dm_hw_check_delta jiffies
 	 */
-	if (!time_after(jiffies, napi->dev->last_rx + dm_hw_check_delta))
+	if (!napi->dev ||
+	    !time_after(jiffies, napi->dev->last_rx + dm_hw_check_delta))
 		return;
 
 	rcu_read_lock();