cgroup: set 'start' with the right value in cgroup_path.

'start' is set to buf + buflen and do the '--' immediately.
Just set it to 'buf + buflen - 1' directly.

Signed-off-by: Tao Ma <boyu.mt@taobao.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Li Zefan <lizefan@huawei.com>
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 3070164..998ab59 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1770,9 +1770,9 @@
 		return 0;
 	}
 
-	start = buf + buflen;
+	start = buf + buflen - 1;
 
-	*--start = '\0';
+	*start = '\0';
 	for (;;) {
 		int len = dentry->d_name.len;