arm64: dma-mapping: Cocci spatch "vma_pages"

Use vma_pages function on vma object instead of explicit computation.
Found by coccinelle spatch "api/vma_pages.cocci"
Reviewed-by: NRobin Murphy <robin.murphy@arm.com>
Signed-off-by: NThomas Meyer <thomas@m3y3r.de>
Signed-off-by: NWill Deacon <will.deacon@arm.com>

arch/arm64/mm/dma-mapping.c

@@ -303,8 +303,7 @@ static int __swiotlb_mmap_pfn(struct vm_area_struct *vma,
 			      unsigned long pfn, size_t size)
 {
 	int ret = -ENXIO;
-	unsigned long nr_vma_pages = (vma->vm_end - vma->vm_start) >>
-					PAGE_SHIFT;
+	unsigned long nr_vma_pages = vma_pages(vma);
 	unsigned long nr_pages = PAGE_ALIGN(size) >> PAGE_SHIFT;
 	unsigned long off = vma->vm_pgoff;