btrfs: Use init_delayed_ref_common in add_delayed_data_ref

Use the newly introduced helper and remove the duplicate code.  No
functional changes.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>