powerpc/iommu: Avoid undefined right shift in iommu_range_alloc()

In iommu_range_alloc() we generate a mask by right shifting ~0,
however if the specified alignment is 0 then we right shift by 64,
which is undefined. UBSAN tells us so:

  UBSAN: Undefined behaviour in ../arch/powerpc/kernel/iommu.c:193:35
  shift exponent 64 is too large for 64-bit type 'long unsigned int'

We can avoid it by instead generating the mask with:

  align_mask = (1ull << align_order) - 1;

That will also generate an undefined shift if align_order is 64 or
greater, but that shouldn't be a problem for a while.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/iommu.c

@@ -190,7 +190,7 @@ static unsigned long iommu_range_alloc(struct device *dev,
 	unsigned int pool_nr;
 	struct iommu_pool *pool;
 
-	align_mask = 0xffffffffffffffffl >> (64 - align_order);
+	align_mask = (1ull << align_order) - 1;
 
 	/* This allocator was derived from x86_64's bit string search */