f2fs: clean up f2fs_ioc_write_checkpoint

Use f2fs_sync_fs to clean up codes in f2fs_ioc_write_checkpoint.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
[Jaegeuk Kim: remove unused err variable]
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>