tuntap: limit the number of flow caches

We create new flow caches when a new flow is identified by tuntap, This may lead
some issues:

- userspace may produce a huge amount of short live flows to exhaust host memory
- the unlimited number of flow caches may produce a long list which increase the
time in the linear searching

Solve this by introducing a limit of total number of flow caches.

Cc: Stephen Hemminger <[e-mail]>
Signed-off-by: Jason Wang <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/tun.c

···
  * the netdevice to be fit in one page. So we can make sure the success of
  * memory allocation. TODO: increase the limit. */
 #define MAX_TAP_QUEUES DEFAULT_MAX_NUM_RSS_QUEUES
+#define MAX_TAP_FLOWS  4096
 
 #define TUN_FLOW_EXPIRE (3 * HZ)
 
···
 	unsigned int numdisabled;
 	struct list_head disabled;
 	void *security;
+	u32 flow_count;
 };
 
 static inline u32 tun_hashfn(u32 rxhash)
···
 		e->queue_index = queue_index;
 		e->tun = tun;
 		hlist_add_head_rcu(&e->hash_link, head);
+		++tun->flow_count;
 	}
 	return e;
 }
···
 		  e->rxhash, e->queue_index);
 	hlist_del_rcu(&e->hash_link);
 	kfree_rcu(e, rcu);
+	--tun->flow_count;
 }
 
 static void tun_flow_flush(struct tun_struct *tun)
···
 		e->updated = jiffies;
 	} else {
 		spin_lock_bh(&tun->lock);
-		if (!tun_flow_find(head, rxhash))
+		if (!tun_flow_find(head, rxhash) &&
+		    tun->flow_count < MAX_TAP_FLOWS)
 			tun_flow_create(tun, head, rxhash, queue_index);
 
 		if (!timer_pending(&tun->flow_gc_timer))