AMD/IOMMU: avoid UB in guest CR3 retrieval

Found by looking for patterns similar to the one Julien did spot in
pci_vtd_quirks(). (Not that it matters much here, considering the code
is dead right now.)

Fixes: 3a7947b690 ("amd-iommu: use a bitfield for DTE")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/drivers/passthrough/amd/iommu_guest.c

@@ -70,7 +70,8 @@ static void guest_iommu_disable(struct guest_iommu *iommu)
 
 static uint64_t get_guest_cr3_from_dte(struct amd_iommu_dte *dte)
 {
-    return ((dte->gcr3_trp_51_31 << 31) | (dte->gcr3_trp_30_15 << 15) |
+    return (((uint64_t)dte->gcr3_trp_51_31 << 31) |
+            (dte->gcr3_trp_30_15 << 15) |
             (dte->gcr3_trp_14_12 << 12)) >> PAGE_SHIFT;
 }