iommu: fix MAX_ORDER usage in __iommu_dma_alloc_pages()

MAX_ORDER is not inclusive: the maximum allocation order buddy allocator
can deliver is MAX_ORDER-1.

Fix MAX_ORDER usage in __iommu_dma_alloc_pages().

Also use GENMASK() instead of hard to read "(2U << order) - 1" magic.

Link: https://lkml.kernel.org/r/20230315113133.11326-10-kirill.shutemov@linux.intel.comSigned-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Acked-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

drivers/iommu/dma-iommu.c

@@ -736,7 +736,7 @@ static struct page **__iommu_dma_alloc_pages(struct device *dev,
 	struct page **pages;
 	unsigned int i = 0, nid = dev_to_node(dev);
 
-	order_mask &= (2U << MAX_ORDER) - 1;
+	order_mask &= GENMASK(MAX_ORDER - 1, 0);
 	if (!order_mask)
 		return NULL;
 
@@ -756,7 +756,7 @@ static struct page **__iommu_dma_alloc_pages(struct device *dev,
 		 * than a necessity, hence using __GFP_NORETRY until
 		 * falling back to minimum-order allocations.
 		 */
-		for (order_mask &= (2U << __fls(count)) - 1;
+		for (order_mask &= GENMASK(__fls(count), 0);
 		     order_mask; order_mask &= ~order_size) {
 			unsigned int order = __fls(order_mask);
 			gfp_t alloc_flags = gfp;