[PATCH] x86_64: Fix off by one in IOMMU check

Fix off by one when checking if the machine has enougn memory to need IOMMU
This caused the IOMMUs to be needlessly enabled for mem=4G

Based on a patch from Jon Mason

Signed-off-by:  <jdmason@us.ibm.com>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>