x86, numa: Reduce minimum fake node size to 32M

This patch changes the minimum fake node size from 64MB to 32MB so it is
possible to test NUMA code at a greater scale on smaller machines
(64 nodes on a 2G machine, 1024 nodes on 32G machine with
CONFIG_NODES_SHIFT=10).

Signed-off-by: David Rientjes <rientjes@google.com>
LKML-Reference: <alpine.DEB.2.00.1012221700590.3701@chino.kir.corp.google.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>