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: Neil Horman <[e-mail]>
Signed-off-by: Xiao Guangrong <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/core/drop_monitor.c

···
 	/*
 	 * 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();