f2fs: allow dio read for LFS mode

commit e57e9ae5b179a6b243c42bf6d9549d1595c27089 upstream.

We can allow dio reads for LFS mode, while doing buffered writes for dio writes.

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