f2fs: sync to upstream

https://git.kernel.org/cgit/linux/kernel/git/jaegeuk/f2fs.git/commit/?h=linux-3.4&id=994642cfdbc08f1eda7ff48504f779cbcc9e3067

Change-Id: I5beb230d4a5dc3ef3913f5bfc4c28b39bcb755b1
diff --git a/Documentation/ABI/testing/sysfs-fs-f2fs b/Documentation/ABI/testing/sysfs-fs-f2fs
index 2c4cc42..e066281d 100644
--- a/Documentation/ABI/testing/sysfs-fs-f2fs
+++ b/Documentation/ABI/testing/sysfs-fs-f2fs
@@ -80,3 +80,9 @@
 Contact:	"Jaegeuk Kim" <jaegeuk@kernel.org>
 Description:
 		 Controls the trimming rate in batch mode.
+
+What:		/sys/fs/f2fs/<disk>/cp_interval
+Date:		October 2015
+Contact:	"Jaegeuk Kim" <jaegeuk@kernel.org>
+Description:
+		 Controls the checkpoint timing.