x86, mm: Use max_low_pfn for ZONE_NORMAL on 64-bit

64-bit has no highmem so max_low_pfn is always the same as
'max_pfn'.

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/1320155902-10424-5-git-send-email-penberg@kernel.org


Signed-off-by: Ingo Molnar <mingo@elte.hu>