btrfs: remove the finish_func argument to btrfs_mark_ordered_io_finished

finish_func is always set to finish_ordered_fn, so remove it and also
the now pointless and somewhat confusingly named
__endio_write_update_ordered wrapper.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David Sterba <dsterba@suse.com>