On x86 truncation cannot occur because config XEN depends on X86_64 ||
(X86_32 && X86_PAE).
On ARM truncation can occur without CONFIG_ARM_LPAE, when the dma
operation involves foreign grants. However in that case the physical
address returned by xen_bus_to_phys is actually invalid (there is no mfn
to pfn tracking for foreign grants on ARM) and it is not used.