[FIX] stock: Allow previous move in chain to reserve for forecast

When chaining multiple moves (like in a multi-step delivery), if
products are reserved on the previous moves of the chain, the products
won't appear as "Reserved" in the forecast report, leading to
inconsistencies between reservation done in stock and the one showed in
the report.

Steps to reproduce :
- Enable multi-step deliveries for this warehouse
- Put enough quantity in stock for a single product
- Create a Sale Order, validate it and the validate the firsts pickings
(i.e. pick/pack transfers)
- Create another Sale Order, validate it. Check that the quantity is
reserved in the first picking (i.e. pick transfer)
- Go to the forecast report, only the first SO will me marked as
"Reserved from Stock", even if both are.

opw-3074572

closes odoo/odoo#107973

X-original-commit: c429c05d
Signed-off-by: Arnold Moyaux (arm) <arm@odoo.com>