KVM: PPC: E500: Implement MMU notifiers

KVM: PPC: E500: Implement MMU notifiers

The e500 target has lived without mmu notifiers ever since it got
introduced, but fails for the user space check on them with hugetlbfs.

So in order to get that one working, implement mmu notifiers in a
reasonably dumb fashion and be happy. On embedded hardware, we almost
never end up with mmu notifier calls, since most people don't overcommit.

Signed-off-by: Alexander Graf <agraf@suse.de>
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v1 -> v2:

  - drop refcount for in-use pages
  - mark pages dirty at mapping time, not unmapping time
  - drop hva_to_memslot usage