x86: add gbpages support to lookup_address

[ tglx@linutronix.de: fix bootup crash on sparse mappings. ]

Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>