x86-64: Combine SRAT regions when possible

... i.e. when the hole between two regions isn't occupied by memory on
another node. This reduces the memory->node table size, thus reducing
cache footprint of lookups, which got increased significantly some
time ago, and things go back to how they were before that change on
the systems I looked at.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
LKML-Reference: <4BCF3230020000780003B3CA@vpn.id2.novell.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>