If the underlying mapping was truncated and someone references the
now-unmapped memory the kernel will enter handle_mm_fault() and will start
instantiating PAGE_SIZE pte's inside the hugepage VMA.  Everything goes
generally pear-shaped.

So trap this in handle_mm_fault().  It adds no overhead to non-hugepage
builds.

Another possible fix would be to not unmap the huge pages at all in truncate
- just anonymise them.

But I think we want full ftruncate semantics for hugepages for management
purposes.