[FIX] stock: partial reserve in MTO

- Create a product with -4.0 Units in stock
- Set the Buy and MTO routes, assign a vendor
- Create a SO for 25 Units
- A PO of 25 Units is created => validate and receive products
- Check availability on the picking linked to the SO

Nothing is reserved, while 21 Units could be reserved.

opw-1850688

Co-authored-by: Nicolas Martinelli <nim@odoo.com>