iommu_gas: speed up augmentation

[dougm]

Summary

Since the tree is balanced, the non-null sibling of a null child can have no descendants, and therefore must have free_down==0. Avoid retrieving the value of free_down from the sibling in such cases.

[dougm]

I can't find a way to exploit this observation about free_down in a way that makes the code smaller, so abandoning.