sysfs: create __ATTR_WO()

This creates the macro __ATTR_WO() for write-only attributes, instead of
having to "open define" them.
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/sysfs.h

@@ -80,6 +80,11 @@ struct attribute_group {
 	.show	= _name##_show,						\
 }
 
+#define __ATTR_WO(_name) {						\
+	.attr	= { .name = __stringify(_name), .mode = S_IWUSR },	\
+	.store	= _name##_store,					\
+}
+
 #define __ATTR_RW(_name) __ATTR(_name, (S_IWUSR | S_IRUGO),		\
 			 _name##_show, _name##_store)