Btrfs: clean up btrfs_ordered_update_i_size

Since we have a good helper entry_end, use it for ordered extent.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ whitespace reformatting ]
Signed-off-by: David Sterba <dsterba@suse.com>