btrfs_ordered_extent->disk_bytenr can be rewritten by the zoned I/O
completion handler, and thus in general is not a good idea to limit I/O
size.  But the maximum bio size calculation can easily be done using the
file_offset fields in the btrfs_ordered_extent and btrfs_bio structures,
so switch to that instead.
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David Sterba <dsterba@suse.com>