[NET]: Set a separate lockdep class for neighbour table's proxy_queue

Otherwise the following calltrace will lead to a wrong
lockdep warning:

  neigh_proxy_process()
    `- lock(neigh_table->proxy_queue.lock);
  arp_redo /* via tbl->proxy_redo */
  arp_process
  neigh_event_ns
  neigh_update
  skb_queue_purge
    `- lock(neighbor->arp_queue.lock);

This is not a deadlock actually, as neighbor table's proxy_queue
and the neighbor's arp_queue are different queues.

Lockdep thinks there is a deadlock as both queues are initialized
with skb_queue_head_init() and thus have a common class.
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -619,6 +619,13 @@ static inline void skb_queue_head_init(struct sk_buff_head *list)
 	list->qlen = 0;
 }
 
+static inline void skb_queue_head_init_class(struct sk_buff_head *list,
+		struct lock_class_key *class)
+{
+	skb_queue_head_init(list);
+	lockdep_set_class(&list->lock, class);
+}
+
 /*
  *	Insert an sk_buff at the start of a list.
  *

net/core/neighbour.c

@@ -1331,6 +1331,8 @@ void neigh_parms_destroy(struct neigh_parms *parms)
 	kfree(parms);
 }
 
+static struct lock_class_key neigh_table_proxy_queue_class;
+
 void neigh_table_init_no_netlink(struct neigh_table *tbl)
 {
 	unsigned long now = jiffies;
@@ -1379,7 +1381,8 @@ void neigh_table_init_no_netlink(struct neigh_table *tbl)
 	init_timer(&tbl->proxy_timer);
 	tbl->proxy_timer.data	  = (unsigned long)tbl;
 	tbl->proxy_timer.function = neigh_proxy_process;
-	skb_queue_head_init(&tbl->proxy_queue);
+	skb_queue_head_init_class(&tbl->proxy_queue,
+			&neigh_table_proxy_queue_class);
 
 	tbl->last_flush = now;
 	tbl->last_rand	= now + tbl->parms.reachable_time * 20;