[NETROM] lockdep: fix false positive

NETROM network devices are virtual network devices encapsulating NETROM
frames into AX.25 which will be sent through an AX.25 device, so form a
special "super class" of normal net devices; split their locks off into a
separate class since they always nest.
Signed-off-by: NRalf Baechle <ralf@linux-mips.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/netrom/af_netrom.c

@@ -65,6 +65,14 @@ static DEFINE_SPINLOCK(nr_list_lock);
 
 static const struct proto_ops nr_proto_ops;
 
+/*
+ * NETROM network devices are virtual network devices encapsulating NETROM
+ * frames into AX.25 which will be sent through an AX.25 device, so form a
+ * special "super class" of normal net devices; split their locks off into a
+ * separate class since they always nest.
+ */
+static struct lock_class_key nr_netdev_xmit_lock_key;
+
 /*
  *	Socket removal during an interrupt is now safe.
  */
@@ -1405,6 +1413,7 @@ static int __init nr_proto_init(void)
 			free_netdev(dev);
 			goto fail;
 		}
+		lockdep_set_class(&dev->_xmit_lock, &nr_netdev_xmit_lock_key);
 		dev_nr[i] = dev;
 	}