ANDROID: KVM: arm64: Skip prefaulting ptes which will be modified later

Block mappings can be split as part of a page table update. When
prefaulting entries during the split, it is pointless to install
valid ptes which will later be modified by the same walk.

At the same time, push the check for pte_is_counted into the
prefault handler, where it logically belongs.

Bug: 278749606
Bug: 308373293
Change-Id: If4599b2860aa62d82ce8db019a8410c2d883de71
Signed-off-by: Keir Fraser <keirf@google.com>