iommu/amd: Fix ineffective error check in nested domain allocation

In the Linux kernel, the following vulnerability has been resolved:

iommu/amd: Fix ineffective error check in nested domain allocation

amd_iommu_pdom_id_alloc() returns an int: a domain ID on success, or the
negative errno from ida_alloc_range() when the ID space is exhausted or
memory is short. amd_iommu_alloc_domain_nested() stores that return value
in gdom_info->hdom_id, which is a u32, and only then tests it:

gdom_info->hdom_id = amd_iommu_pdom_id_alloc();
if (gdom_info->hdom_id <= 0) {

The assignment discards the sign, so -ENOSPC becomes 0xffffffe4 and the
test never fires. The nested domain is then set up with a host domain ID
that was never allocated, instead of the allocation failing with -ENOSPC.

Keep the value in an int, test it there, and store it only once it is
known to be valid, which is what the other amd_iommu_pdom_id_alloc()
callers already do.