kmemcheck: use kmemcheck_pte_lookup() instead of open-coding it

Lets use kmemcheck_pte_lookup() in kmemcheck_fault() instead of
open-coding it there.

Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Vegard Nossum <vegard.nossum@gmail.com>