ext4: fix off-by-one error when writing back pages before dio read

The 'lend' argument of filemap_write_and_wait_range() is inclusive, so
we need to subtract 1 from pos + count.

Note that 'count' is guaranteed to be nonzero since
ext4_file_read_iter() returns early when given a 0 count.

Fixes: 16c54688 ("ext4: Allow parallel DIO reads")
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>

fs/ext4/inode.c

@@ -3713,7 +3713,7 @@ static ssize_t ext4_direct_IO_read(struct kiocb *iocb, struct iov_iter *iter)
 	 */
 	inode_lock_shared(inode);
 	ret = filemap_write_and_wait_range(mapping, iocb->ki_pos,
-					   iocb->ki_pos + count);
+					   iocb->ki_pos + count - 1);
 	if (ret)
 		goto out_unlock;
 	ret = __blockdev_direct_IO(iocb, inode, inode->i_sb->s_bdev,