exfat: fix zero the unwritten part for dio read

For dio read, bio will be leave in flight when a successful partial
aio read have been setup, blockdev_direct_IO() will return
-EIOCBQUEUED. In the case, iter->iov_offset will be not advanced,
the oops reported by syzbot will occur if revert iter->iov_offset
with iov_iter_revert(). The unwritten part had been zeroed by aio
read, so there is no need to zero it in dio read.

Reported-by:  <syzbot+fd404f6b03a58e8bc403@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=fd404f6b03a58e8bc403


Fixes: 11a347fb ("exfat: change to get file size from DataLength")
Signed-off-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>