f2fs: cleanup the disk level filename updating

As discuss with Jaegeuk and Chao,
"Once checkpoint is done, f2fs doesn't need to update there-in filename at all."

The disk-level filename is used only one case,
1. create a file A under a dir
2. sync A
3. godown
4. umount
5. mount (roll_forward)

Only the rename/cross_rename changes the filename, if it happens,
a. between step 1 and 2, the sync A will caused checkpoint, so that,
   the roll_forward at step 5 never happens.
b. after step 2, the roll_forward happens, file A will roll forward
   to the result as after step 1.

So that, any updating the disk filename is useless, just cleanup it.

Signed-off-by: Kinglong Mee <kinglongmee@gmail.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>