s390/mm: add missing pfn_to_kaddr helper

kasan common code uses pfn_to_kaddr, which is defined by many other
architectures. Adding it as well to avoid a build error.

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>