[FIX] mrp: cancel leftover moves when create backorder

Previously, when underconsumption occured, we split the move (in
post_inventory). And if backorder, the moves not done would be linked to
the new backorder MO (when backorder MO is created).
After 8883c06a, we create backorder MOs
before _post_inventory, making it so the moves not done will not be
linked to the backorder MOs and reserved qtys are not released.
To fix, we set cancel_backorder to be true to cancel all the leftover
moves and release the reserved qty.

Task-2697611

closes odoo/odoo#80271

closes odoo/odoo#110990

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