x86, mm: use pfn_range_is_mapped() with gart

We are going to map ram only, so under max_low_pfn_mapped,
between 4g and max_pfn_mapped does not mean mapped at all.

Use pfn_range_is_mapped() directly.
Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Link: http://lkml.kernel.org/r/1353123563-3103-14-git-send-email-yinghai@kernel.orgSigned-off-by: H. Peter Anvin <hpa@linux.intel.com>

arch/x86/kernel/amd_gart_64.c

@@ -768,10 +768,9 @@ int __init gart_iommu_init(void)
 	aper_base	= info.aper_base;
 	end_pfn		= (aper_base>>PAGE_SHIFT) + (aper_size>>PAGE_SHIFT);
 
-	if (end_pfn > max_low_pfn_mapped) {
-		start_pfn = (aper_base>>PAGE_SHIFT);
+	start_pfn = PFN_DOWN(aper_base);
+	if (!pfn_range_is_mapped(start_pfn, end_pfn))
 		init_memory_mapping(start_pfn<<PAGE_SHIFT, end_pfn<<PAGE_SHIFT);
-	}
 
 	pr_info("PCI-DMA: using GART IOMMU.\n");
 	iommu_size = check_iommu_size(info.aper_base, aper_size);