driver core: make sysfs_dev_block_kobj static

Nothing outside of drivers/base/core.c uses sysfs_dev_block_kobj, so
make it static and document what it is used for so we remember it the
next time we touch it 15 years from now.
Acked-by: Rafael J. Wysocki <rafael@kernel.org>
Link: https://lore.kernel.org/r/20230331093318.82288-6-gregkh@linuxfoundation.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/base/core.c

@@ -2256,7 +2256,9 @@ int (*platform_notify)(struct device *dev) = NULL;
 int (*platform_notify_remove)(struct device *dev) = NULL;
 static struct kobject *dev_kobj;
 struct kobject *sysfs_dev_char_kobj;
-struct kobject *sysfs_dev_block_kobj;
+
+/* /sys/dev/block */
+static struct kobject *sysfs_dev_block_kobj;
 
 static DEFINE_MUTEX(device_hotplug_lock);
 

include/linux/device/class.h

@@ -76,8 +76,6 @@ struct class_dev_iter {
 	const struct device_type	*type;
 };
 
-extern struct kobject *sysfs_dev_block_kobj;
-
 int __must_check class_register(struct class *class);
 void class_unregister(const struct class *class);
 bool class_is_registered(const struct class *class);