KVM: PPC: Book3S HV: Handle 1GB pages in radix page fault handler

This adds code to the radix hypervisor page fault handler to handle the
case where the guest memory is backed by 1GB hugepages, and put them
into the partition-scoped radix tree at the PUD level.  The code is
essentially analogous to the code for 2MB pages.  This also rearranges
kvmppc_create_pte() to make it easier to follow.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>