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: NMartin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: NVasily Gorbik <gor@linux.ibm.com>
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/include/asm/pgtable.h

@@ -341,6 +341,8 @@ static inline int is_module_addr(void *addr)
 #define PTRS_PER_P4D	_CRST_ENTRIES
 #define PTRS_PER_PGD	_CRST_ENTRIES
 
+#define MAX_PTRS_PER_P4D	PTRS_PER_P4D
+
 /*
  * Segment table and region3 table entry encoding
  * (R = read-only, I = invalid, y = young bit):