powerpc/iommu: Fix iommu_table_in_use for a small default DMA window case

The existing iommu_table_in_use() helper checks if the kernel is using
any of TCEs. There are some reserved TCEs:
1) the very first one if DMA window starts from 0 to avoid having a zero
but still valid DMA handle;
2) it_reserved_start..it_reserved_end to exclude MMIO32 window in case
the default window spans across that - this is the default for the first
DMA window on PowerNV.

When 1) is the case and 2) is not the helper does not skip 1) and returns
wrong status.

This only seems occurring when passing through a PCI device to a nested
guest (not something we support really well) so it has not been seen
before.

This fixes the bug by adding a special case for no MMIO32 reservation.

Fixes: 3c33066a ("powerpc/kernel/iommu: Add new iommu_table_in_use() helper")
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220714081119.3714605-1-aik@ozlabs.ru