kobject: explain what kobject's sd field is

(More) unclear, especially name-wise, after sysfs_dirent became
kernfs_node.
Signed-off-by: NUlf Magnusson <ulfalizer@gmail.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/kobject.h

@@ -66,7 +66,7 @@ struct kobject {
 	struct kobject		*parent;
 	struct kset		*kset;
 	struct kobj_type	*ktype;
-	struct kernfs_node	*sd;
+	struct kernfs_node	*sd; /* sysfs directory entry */
 	struct kref		kref;
 #ifdef CONFIG_DEBUG_KOBJECT_RELEASE
 	struct delayed_work	release;