[driver model] Add device_for_each_child iterator.

From Mike Anderson: 

I have been using it on an outstanding patch for scsi_set_host_offline. It
appears to work fine in my testing.

drivers/base/core.c

@@ -343,11 +343,40 @@ void device_unregister(struct device * dev)
 	put_device(dev);
 }
 
+/**
+ *	device_for_each_child - device child iterator.
+ *	@dev:	parent struct device.
+ *	@data:	data for the callback.
+ *	@fn:	function to be called for each device.
+ *
+ *	Iterate over @dev's child devices, and call @fn for each,
+ *	passing it @data. 
+ *
+ *	We check the return of @fn each time. If it returns anything
+ *	other than 0, we break out and return that value.
+ */
+int device_for_each_child(struct device * dev, void * data,
+		     int (*fn)(struct device *, void *))
+{
+	struct device * child;
+	int error = 0;
+
+	down_read(&devices_subsys.rwsem);
+	list_for_each_entry(child,&dev->children,node) {
+		if((error = fn(child,data)))
+			break;
+	}
+	up_read(&devices_subsys.rwsem);
+	return error;
+}
+
 int __init devices_init(void)
 {
 	return subsystem_register(&devices_subsys);
 }
 
+EXPORT_SYMBOL(device_for_each_child);
+
 EXPORT_SYMBOL(device_initialize);
 EXPORT_SYMBOL(device_add);
 EXPORT_SYMBOL(device_register);

include/linux/device.h

@@ -303,6 +303,8 @@ extern void device_unregister(struct device * dev);
 extern void device_initialize(struct device * dev);
 extern int device_add(struct device * dev);
 extern void device_del(struct device * dev);
+extern int device_for_each_child(struct device *, void *,
+		     int (*fn)(struct device *, void *));
 
 /*
  * Manual binding of a device to driver. See drivers/base/bus.c