iommu/s390: Fix NULL dereference in iova_to_phys() with ZPCI_TABLE_TYPE_RFX

In the Linux kernel, the following vulnerability has been resolved: iommu/s390: Fix NULL dereference in iova_to_phys() with ZPCI_TABLE_TYPE_RFX When using a 5-level translation table via ZPCI_TABLE_TYPE_RFX get_rso_from_iova() returns NULL when the region-first entry is invalid. Yet in get_rto_from_iova() the region-second origin rso is not checked to be non-NULL before accessing rso[rsx] leading to a NULL pointer dereference instead of a NULL return when iova_to_phys() is called on a unmapped IOVA. Fix this by adding the missing NULL check.