[PATCH] x86_64: Fix sparse mem

Fix up booting with sparse mem enabled. Otherwise it would just
cause an early PANIC at boot.

Signed-off-by: Bob Picco <bob.picco@hp.com>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>