driver model: protect drivers' device list accesses with bus's rwsem.

Drivers must belong to bus, and each bus has an rwsem. Instead of mucking with
the device_lock spinlock, and dropping it on each iteration of the loop, we take 
the bus's lock (read, so multiple drivers can access their list at the same time)
around the entire walk of the list.

drivers/base/driver.c

@@ -16,26 +16,23 @@ int driver_for_each_dev(struct device_driver * drv, void * data,
 			int (*callback)(struct device *, void * ))
 {
 	struct list_head * node;
-	struct device * prev = NULL;
 	int error = 0;
 
-	get_driver(drv);
-	spin_lock(&device_lock);
-	list_for_each(node,&drv->devices) {
-		struct device * dev = get_device_locked(to_dev(node));
-		if (dev) {
-			spin_unlock(&device_lock);
-			error = callback(dev,data);
-			if (prev)
-				put_device(prev);
-			prev = dev;
-			spin_lock(&device_lock);
-			if (error)
-				break;
+	drv = get_driver(drv);
+	if (drv) {
+		down_read(&drv->bus->rwsem);
+		list_for_each(node,&drv->devices) {
+			struct device * dev = get_device(to_dev(node));
+			if (dev) {
+				error = callback(dev,data);
+				put_device(dev);
+				if (error)
+					break;
+			}
 		}
+		up_read(&drv->bus->rwsem);
+		put_driver(drv);
 	}
-	spin_unlock(&device_lock);
-	put_driver(drv);
 	return error;
 }