s390/kasan: add option for 4-level paging support

By default 3-level paging is used when the kernel is compiled with
kasan support. Add 4-level paging option to support systems with more
then 3TB of physical memory and to cover 4-level paging specific code
with kasan as well.

Reviewed-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>