btrfs: make btrfs_finish_ordered_extent() return void

Currently btrfs_finish_ordered_extent() returns a boolean indicating if
the ordered extent was added to the work queue for completion, but none
of its callers cares about it, so make it return void.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>