x86, mm: Use MAX_DMA_PFN for ZONE_DMA on 32-bit

Use MAX_DMA_PFN which represents the 16 MB ISA DMA limit on
32-bit x86 just like we do on 64-bit.
Acked-by: Tejun Heo <tj@kernel.org>
Acked-by: Yinghai Lu <yinghai@kernel.org>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>
Link: http://lkml.kernel.org/r/1320155902-10424-1-git-send-email-penberg@kernel.orgSigned-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/mm/init_32.c

@@ -679,8 +679,7 @@ static void __init zone_sizes_init(void)
 	unsigned long max_zone_pfns[MAX_NR_ZONES];
 	memset(max_zone_pfns, 0, sizeof(max_zone_pfns));
 #ifdef CONFIG_ZONE_DMA
-	max_zone_pfns[ZONE_DMA] =
-		virt_to_phys((char *)MAX_DMA_ADDRESS) >> PAGE_SHIFT;
+	max_zone_pfns[ZONE_DMA] = MAX_DMA_PFN;
 #endif
 	max_zone_pfns[ZONE_NORMAL] = max_low_pfn;
 #ifdef CONFIG_HIGHMEM