s390/mm: correct pgtable_bytes on page table downgrade

The downgrade of a page table from 3 levels to 2 levels for a 31-bit compat
process removes a pmd table which has to be counted against pgtable_bytes.
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/mm/pgalloc.c

@@ -131,6 +131,7 @@ void crst_table_downgrade(struct mm_struct *mm)
 	}
 
 	pgd = mm->pgd;
+	mm_dec_nr_pmds(mm);
 	mm->pgd = (pgd_t *) (pgd_val(*pgd) & _REGION_ENTRY_ORIGIN);
 	mm->context.asce_limit = _REGION3_SIZE;
 	mm->context.asce = __pa(mm->pgd) | _ASCE_TABLE_LENGTH |