[FIX] stock: forecast on multi-step manufacturing

When in a multi-step manufacturing scenario, if there is more than enough to complete one MO, the forecast shown is wrong as it looks like it is trying to reserve more than once the quantities in stock.

Steps to reproduce:

- Activate multi-step routes and set manufacturing in 3 steps (warehouse settings)
- Create a product (Component) with UOM kg and add 200kg in stock.
- Create another product (Final product) with a BoM.
- On the BoM, set that to manufacture 100 units, 51700g should be consumed.
- Create a MO for 210 units and confirm.
- Check the forecast of the Component.

OPW-3231092

closes odoo/odoo#117455

Signed-off-by: Arnold Moyaux (arm) <arm@odoo.com>