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>

fs/exfat/inode.c

@@ -501,7 +501,7 @@ static ssize_t exfat_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
 	struct inode *inode = mapping->host;
 	struct exfat_inode_info *ei = EXFAT_I(inode);
 	loff_t pos = iocb->ki_pos;
-	loff_t size = iocb->ki_pos + iov_iter_count(iter);
+	loff_t size = pos + iov_iter_count(iter);
 	int rw = iov_iter_rw(iter);
 	ssize_t ret;
 
@@ -525,11 +525,10 @@ static ssize_t exfat_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
 	 */
 	ret = blockdev_direct_IO(iocb, inode, iter, exfat_get_block);
 	if (ret < 0) {
-		if (rw == WRITE)
+		if (rw == WRITE && ret != -EIOCBQUEUED)
 			exfat_write_failed(mapping, size);
 
-		if (ret != -EIOCBQUEUED)
-			return ret;
+		return ret;
 	} else
 		size = pos + ret;