[S390] missing return in page_table_alloc_pgste

Fix the following compile warning for !CONFIG_PGSTE:

  CC      arch/s390/mm/pgtable.o
arch/s390/mm/pgtable.c: In function ‘page_table_alloc_pgste’:
arch/s390/mm/pgtable.c:531:1: warning: no return statement in function returning non-void [-Wreturn-type]
Signed-off-by: Jan Glauber <jang@linux.vnet.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>

arch/s390/mm/pgtable.c

@@ -528,6 +528,7 @@ static inline void page_table_free_pgste(unsigned long *table)
 static inline unsigned long *page_table_alloc_pgste(struct mm_struct *mm,
 						    unsigned long vmaddr)
 {
+	return NULL;
 }
 
 static inline void page_table_free_pgste(unsigned long *table)