f2fs: correct i_size change for atomic writes

We need to make sure i_size doesn't change until atomic write commit is
successful and restore it when commit is failed.

Signed-off-by: Daeho Jeong <daehojeong@google.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>