ARM: dma-mapping: fix error path for memory allocation failure

This patch fixes incorrect check in error path. When the allocation of
first page fails, the kernel ops appears due to accessing -1 element of
the pages array.
Reported-by: NSylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: NMarek Szyprowski <m.szyprowski@samsung.com>

arch/arm/mm/dma-mapping.c

@@ -961,7 +961,7 @@ static struct page **__iommu_alloc_buffer(struct device *dev, size_t size, gfp_t
 
 	return pages;
 error:
-	while (--i)
+	while (i--)
 		if (pages[i])
 			__free_pages(pages[i], 0);
 	if (array_size <= PAGE_SIZE)