drop_monitor: Document scope of spinlock

While 'per_cpu_dm_data' is a per-CPU variable, its 'skb' and
'send_timer' fields can be accessed concurrently by the CPU sending the
netlink notification to user space from the workqueue and the CPU
tracing kfree_skb(). This spinlock is meant to protect against that.

Document its scope and suppress the checkpatch message "spinlock_t
definition without comment".
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/drop_monitor.c

@@ -52,7 +52,7 @@ static int trace_state = TRACE_OFF;
 static DEFINE_MUTEX(net_dm_mutex);
 
 struct per_cpu_dm_data {
-	spinlock_t		lock;
+	spinlock_t		lock;	/* Protects 'skb' and 'send_timer' */
 	struct sk_buff		*skb;
 	struct work_struct	dm_alert_work;
 	struct timer_list	send_timer;