[FIX] stock: auto assign backorder

When a backorder is created, the quantities are not reserved. This is an
issue: the module should try to directly assign these quantities.

(Revert of 6b15eab7)

OPW-2658469

closes odoo/odoo#79692

Signed-off-by: William Henrotin (whe) <whe@odoo.com>