Back Order customer notification does not show

@andrewbess is already working on this.

Since Apr 29, 2021.

### Preconditions (*)

Magento 2.4.1
MSI

### Steps to reproduce (*)

1. Create a product, add to source
2. Put QTY as -1 and Out of Stock Threshold as whatever you want (i.e. -20)
3. Add one to cart
4. Go to checkout page or cart page

### Expected result (*)

1. A message displays that says "We don't have as many quantity as you requested, but we'll back order the remaining ___"

### Actual result (*)

1. No message is displayed.

It seems that https://github.com/magento/inventory/pull/3143 created by @arnobsh broke backorder messaging in some cases. It doesn't take into account $stockItemData, and thus if the stock item qty is -1 and out of stock threshold is -20, it ends up with displayQty = 0, and thus doesn't display a backorder message. Salable Quantity is > 0, but stock available data is < 0 and thus should be displaying the stock available data's number as backorder number.

My suggestion would be to add
```
if ($backOrderQty < 0 && $requestedQty < $salableQty) {
$displayQty = $requestedQty;
}
```

to getDisplayQty