Cannot complete payment via admin

[pmontury]

Sylius version affected: 1.12.14

Description
Cannot complete an order payment via admin

Steps to reproduce
On a stock tracked product (onHand = 3 onHold = 0) enter an order for a quantity of 2
After the checkout is completed we have onHand = 3 and onHold = 2 (so far so good)
When trying to complete the order payment via the admin we get the error 'The payment cannot be completed due to insufficient stock of the product.

Possible Solution
The payment via admin triggers the PaymentPreCompleteListener which calls the availabityChecker isStockSufficient method.
isStocksufficient returns false because the order quantity is greater than the variant onHand - onHold quantities.
That seems wrong at that step since the order quantity is already accounted for in the variant onHold quantity and that the check should be done against onHold quantity.