software node: Handle software node injection to an existing device properly

The function software_node_notify() - the function that creates
and removes the symlinks between the node and the device - was
called unconditionally in device_add_software_node() and
device_remove_software_node(), but it needs to be called in
those functions only in the special case where the node is
added to a device that has already been registered.

This fixes NULL pointer dereference that happens if
device_remove_software_node() is used with device that was
never registered.

Fixes: b622b24519f5 ("software node: Allow node addition to already existing device")
Reported-and-tested-by: Dominik Brodowski <[e-mail]>
Reviewed-by: Andy Shevchenko <[e-mail]>
Signed-off-by: Heikki Krogerus <[e-mail]>
Signed-off-by: Rafael J. Wysocki <[e-mail]>

drivers/base/swnode.c

···
 	}
 
 	set_secondary_fwnode(dev, &swnode->fwnode);
-	software_node_notify(dev, KOBJ_ADD);
+
+	/*
+	 * If the device has been fully registered by the time this function is
+	 * called, software_node_notify() must be called separately so that the
+	 * symlinks get created and the reference count of the node is kept in
+	 * balance.
+	 */
+	if (device_is_registered(dev))
+		software_node_notify(dev, KOBJ_ADD);
 
 	return 0;
 }
···
 	if (!swnode)
 		return;
 
-	software_node_notify(dev, KOBJ_REMOVE);
+	if (device_is_registered(dev))
+		software_node_notify(dev, KOBJ_REMOVE);
 	set_secondary_fwnode(dev, NULL);
 	kobject_put(&swnode->kobj);
 }
···
 
 	switch (action) {
 	case KOBJ_ADD:
-		ret = sysfs_create_link_nowarn(&dev->kobj, &swnode->kobj,
-					       "software_node");
+		ret = sysfs_create_link(&dev->kobj, &swnode->kobj, "software_node");
 		if (ret)
 			break;