x86: remove (null) in /sys kernel_page_tables

Impact: cleanup

%p prints out 0x000000000000000 as (null)
so use %lx instead.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
LKML-Reference: <49E43282.1090607@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>