s390/mm: use TASK_MAX_SIZE where applicable

To improve readability we can use TASK_MAX_SIZE when we just check for the
upper limit.  All places explicitly dealing with 3 vs 4 level pgtables
were left unchanged.

Signed-off-by: Dominik Dingel <dingel@linux.vnet.ibm.com>
Reviewed-By: Sascha Silbe <silbe@linux.vnet.ibm.com>

arch/s390/mm/pgtable.c

@@ -55,7 +55,7 @@ int crst_table_upgrade(struct mm_struct *mm, unsigned long limit)
 	unsigned long entry;
 	int flush;
 
-	BUG_ON(limit > (1UL << 53));
+	BUG_ON(limit > TASK_MAX_SIZE);
 	flush = 0;
 repeat:
 	table = crst_table_alloc(mm);