f2fs: revert segment allocation for direct IO

commit 6ae1be13e85f4c42c8ca371fda50ae39eebbfd96 upstream.

Now we don't need to be too much careful about storage alignment for dio, since
its speed becomes quite fast and we'd better avoid any misalignment first.

Revert: 38aa0889 (f2fs: align direct_io'ed data to section)

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