powerpc/mm/iommu: allow large IOMMU page size only for hugetlb backing

THP pages can get split during different code paths.  An incremented
reference count does imply we will not split the compound page.  But the
pmd entry can be converted to level 4 pte entries.  Keep the code
simpler by allowing large IOMMU page size only if the guest ram is
backed by hugetlb pages.

Link: http://lkml.kernel.org/r/20190114095438.32470-6-aneesh.kumar@linux.ibm.com
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Cc: Alexey Kardashevskiy <aik@ozlabs.ru>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: David Gibson <david@gibson.dropbear.id.au>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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