Impacted versions: 9.0

Steps to reproduce:

* Create an MTO product with 
  * Product Type: Stockable Product
  * Routes: "Buy" and "Make to Order"
* Update Qty on Hand to any quantity (i.e, 2).
* Create a sales order with this product and a number <= to the one in the step before.
* Confirm the sales order.

Current behavior:

The created delivery picking appears in state "Reserved" and the current quantity on stock has been reserved, allowing you to transfer that stock.

Expected behavior:

The created delivery picking should be in "Waiting another operation", or at least in "Waiting availability" until the purchase order is confirmed (if you have defined a supplier for the product and the procurement creates the purchase order), which seems to be the current behavior when there's no stock available (is this also correct, as it allows to manually reserve?).
addons/stock/stock.py
qtyassign_cmp = float_compare(qty_to_assign, 0.0, precision_rounding=rounding)
            if prod2move_ids.get(product_id):
                while prod2move_ids[product_id] and qtyassign_cmp > 0:
                    qty_on_link = _create_link_for_index(operation_id, 0, product_id, qty_to_assign, quant_id=False)
                    qty_to_assign -= qty_on_link
                    qtyassign_cmp = float_compare(qty_to_assign, 0.0, precision_rounding=rounding)
            return qtyassign_cmp == 0
        uom_obj = self.pool.get('product.uom')
        package_obj = self.pool.get('stock.quant.package')
        quant_obj = self.pool.get('stock.quant')
        link_obj = self.pool.get('stock.move.operation.link')
        quants_in_package_done = set()
        prod2move_ids = {}
        still_to_do = []
        
        moves = sorted([x for x in picking.move_lines if x.state not in ('done', 'cancel')], key=lambda x: (((x.state == 'assigned') and -2 or 0) + (x.partially_available and -1 or 0)))
        for move in moves:
            if not prod2move_ids.get(move.product_id.id):
                prod2move_ids[move.product_id.id] = [{'move': move, 'remaining_qty': move.product_qty}]
            else:
                prod2move_ids[move.product_id.id].append({'move': move, 'remaining_qty': move.product_qty})
        need_rereserve = False
        
        operations = picking.pack_operation_ids
        operations = sorted(operations, key=lambda x: ((x.package_id and not x.product_id) and -4 or 0) + (x.package_id and -2 or 0) + (x.pack_lot_ids and -1 or 0))
        
        links = link_obj.search(cr, uid, [('operation_id', 'in', [x.id for x in operations])], context=context)
        if links:
            link_obj.unlink(cr, uid, links, context=context)