decnet: add RTNL lock when reading address list

Add missing locking in the case of auto binding to the
default device. The address list might change while this code is looking
at the list.

Compile tested only, I am not a decnet user.
Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/decnet/dn_dev.c

@@ -828,13 +828,17 @@ static int dn_dev_get_first(struct net_device *dev, __le16 *addr)
 	struct dn_dev *dn_db = (struct dn_dev *)dev->dn_ptr;
 	struct dn_ifaddr *ifa;
 	int rv = -ENODEV;
+
 	if (dn_db == NULL)
 		goto out;
+
+	rtnl_lock();
 	ifa = dn_db->ifa_list;
 	if (ifa != NULL) {
 		*addr = ifa->ifa_local;
 		rv = 0;
 	}
+	rtnl_unlock();
 out:
 	return rv;
 }