[PATCH] x86_64: Fix zero mcfg entry workaround on x86-64

I broke this earlier when moving the patch from i386 to x86-64.
Need to return the virtual address here, not the physical address.
This fixes some boot time crashes on x86-64.

Cc: gregkh@suse.de

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