cgroup: implement CFTYPE_NO_PREFIX

When cgroup files are created, cgroup core automatically prepends the
name of the subsystem as prefix.  This patch adds CFTYPE_NO_ which
disables the automatic prefix.  This is to work around historical
baggages and shouldn't be used for new files.

This will be used to move "cgroup.event_control" from cgroup core to
memcg.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Glauber Costa <glommer@gmail.com>
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 21ba298..3561d30 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -411,6 +411,7 @@
 	CFTYPE_ONLY_ON_ROOT	= (1 << 0),	/* only create on root cgrp */
 	CFTYPE_NOT_ON_ROOT	= (1 << 1),	/* don't create on root cgrp */
 	CFTYPE_INSANE		= (1 << 2),	/* don't create if sane_behavior */
+	CFTYPE_NO_PREFIX	= (1 << 3),	/* (DON'T USE FOR NEW FILES) no subsys prefix */
 };
 
 #define MAX_CFTYPE_NAME		64