f2fs: update f2fs document regarding to fsync_mode

This patch adds missing fsync_mode entry in f2fs document.

Fixes: 04485987 ("f2fs: introduce async IPU policy")
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

Documentation/filesystems/f2fs.txt

@@ -334,7 +334,8 @@ Files in /sys/fs/f2fs/<devname>
                               updates in f2fs. User can set:
                                0x01: F2FS_IPU_FORCE, 0x02: F2FS_IPU_SSR,
                                0x04: F2FS_IPU_UTIL,  0x08: F2FS_IPU_SSR_UTIL,
-                               0x10: F2FS_IPU_FSYNC, 0x40: F2FS_IPU_NOCACHE.
+                               0x10: F2FS_IPU_FSYNC, 0x20: F2FS_IPU_ASYNC,
+                               0x40: F2FS_IPU_NOCACHE.
                               Refer segment.h for details.
 
  min_ipu_util                 This parameter controls the threshold to trigger

fs/f2fs/segment.h

@@ -607,9 +607,10 @@ static inline int utilization(struct f2fs_sb_info *sbi)
  *                     threashold,
  * F2FS_IPU_FSYNC - activated in fsync path only for high performance flash
  *                     storages. IPU will be triggered only if the # of dirty
- *                     pages over min_fsync_blocks.
+ *                     pages over min_fsync_blocks. (=default option)
+ * F2FS_IPU_ASYNC - do IPU given by asynchronous write requests.
  * F2FS_IPU_NOCACHE - disable IPU bio cache.
- * F2FS_IPUT_DISABLE - disable IPU. (=default option)
+ * F2FS_IPUT_DISABLE - disable IPU. (=default option in LFS mode)
  */
 #define DEF_MIN_IPU_UTIL	70
 #define DEF_MIN_FSYNC_BLOCKS	8