net: add CONFIG_PCPU_DEV_REFCNT

I was working on a syzbot issue, claiming one device could not be
dismantled because its refcount was -1

unregister_netdevice: waiting for sit0 to become free. Usage count = -1

It would be nice if syzbot could trigger a warning at the time
this reference count became negative.

This patch adds CONFIG_PCPU_DEV_REFCNT options which defaults
to per cpu variables (as before this patch) on SMP builds.

v2: free_dev label in alloc_netdev_mqs() is moved to avoid
    a compiler warning (-Wunused-label), as reported
    by kernel test robot <lkp@intel.com>

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -2092,7 +2092,12 @@ struct net_device {
 	u32                     proto_down_reason;
 
 	struct list_head	todo_list;
+
+#ifdef CONFIG_PCPU_DEV_REFCNT
 	int __percpu		*pcpu_refcnt;
+#else
+	refcount_t		dev_refcnt;
+#endif
 
 	struct list_head	link_watch_list;
 
@@ -4044,7 +4049,11 @@ void netdev_run_todo(void);
  */
 static inline void dev_put(struct net_device *dev)
 {
+#ifdef CONFIG_PCPU_DEV_REFCNT
 	this_cpu_dec(*dev->pcpu_refcnt);
+#else
+	refcount_dec(&dev->dev_refcnt);
+#endif
 }
 
 /**
@@ -4055,7 +4064,11 @@ static inline void dev_put(struct net_device *dev)
  */
 static inline void dev_hold(struct net_device *dev)
 {
+#ifdef CONFIG_PCPU_DEV_REFCNT
 	this_cpu_inc(*dev->pcpu_refcnt);
+#else
+	refcount_inc(&dev->dev_refcnt);
+#endif
 }
 
 /* Carrier loss detection, dial on demand. The functions netif_carrier_on