x86: cpa, fix lookup_address

lookup_address() returns a wrong level and a wrong pointer to a non
existing pte, when pmd or pud entries are marked !present. This
happens for example due to boot time mapping of GART into the low
memory space.

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