s390/vmem: fix vmem_add_range for 4-level paging

The kernel currently crashes if 4-level paging is used. Add missing
p4d_populate for just allocated pud entry.

Fixes: 3e0d3e40 ("s390/vmem: consolidate vmem_add_range() and vmem_remove_range()")
Reviewed-by: NGerald Schaefer <gerald.schaefer@linux.ibm.com>
Signed-off-by: NVasily Gorbik <gor@linux.ibm.com>

arch/s390/mm/vmem.c

@@ -402,6 +402,7 @@ static int modify_p4d_table(pgd_t *pgd, unsigned long addr, unsigned long end,
 			pud = vmem_crst_alloc(_REGION3_ENTRY_EMPTY);
 			if (!pud)
 				goto out;
+			p4d_populate(&init_mm, p4d, pud);
 		}
 		ret = modify_pud_table(p4d, addr, next, add, direct);
 		if (ret)