x86, mm: Allow ZONE_DMA to be configurable

ZONE_DMA is unnecessary for a large number of machines that do not
require less than 32-bit DMA addressing, e.g. ISA legacy DMA or PCI
cards with a restricted DMA address mask.

This patch allows users to disable ZONE_DMA for x86 if they know they
will not be using such devices with their kernel.

This prevents the VM from unnecessarily reserving a ratio of memory
(defaulting to 1/256th of system capacity) with lowmem_reserve_ratio
for such allocations when it will never be used.

Signed-off-by: David Rientjes <rientjes@google.com>
Link: http://lkml.kernel.org/r/alpine.DEB.2.00.1105161353560.4353@chino.kir.corp.google.com


Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>