btrfs: factor out a btrfs_queue_ordered_fn helper

Factor out a helper to queue up an ordered_extent completion in a work
queue.  This new helper will later be used complete an ordered_extent
without first doing a lookup.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>