powerpc/powernv: Fix iommu table size calculation hook for small tables

When the userspace requests a small TCE table (which takes less than
the system page size) and more than 1 TCE level, the existing code
returns a single page size which is a bug as each additional TCE level
requires at least one page and this is what
pnv_pci_ioda2_table_alloc_pages() does. And we end up seeing
WARN_ON(!ret && ((*ptbl)->it_allocated_size != table_size))
in drivers/vfio/vfio_iommu_spapr_tce.c.

This replaces incorrect _ALIGN_UP() (which aligns zero up to zero) with
max_t() to fix the bug.

Besides removing WARN_ON(), there should be no other changes in
behaviour.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/platforms/powernv/pci-ioda.c

@@ -2440,7 +2440,8 @@ static unsigned long pnv_pci_ioda2_get_table_size(__u32 page_shift,
 
 		tce_table_size /= direct_table_size;
 		tce_table_size <<= 3;
-		tce_table_size = _ALIGN_UP(tce_table_size, direct_table_size);
+		tce_table_size = max_t(unsigned long,
+				tce_table_size, direct_table_size);
 	}
 
 	return bytes;