s390: introduce MAX_PTRS_PER_P4D

Kasan common code requires MAX_PTRS_PER_P4D definition, which in case
of s390 is always PTRS_PER_P4D.

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>