amd_idpgtbl: use page iterator instead of vm_page_next

[dougm]

Summary

Use VM_RADIX_FORALL instead of vm_page_next in a loop in amdiommu_domain_free_pgtbl.

[kib]

Now that I look at the actual use of VM_RADIX_* macros, I think that it is better to require callers to take the pointer to iterator instead of doing it in macro itself. I.e. it would be VM_RADIX_FORALL(m, &pages); which is more natural C.