[FIX] mrp: fix qty_available for kit with sub-kits

On this situation:
* P1 consumable product, kit, with components P2 and P3 in its BoM using
  1 of each
* P2 comsumable product, kit, with component P3 using 1 in the BoM
* P3 storable product, 10 units in stock

Before:
The qyt_available for P1 is 10. That's incorrect: to assemble one P1 we
need precisely two of P3s, one for P2 BoM then an extra for P1 BoM.

After:
The qyt_available for P1 is 5.

closes odoo/odoo#120752

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