!5220 powerpc/mm: Fix null-pointer dereference in pgtable_cache_add

Merge Pull Request from: @ci-robot 
 
PR sync from: Peng Zhang <zhangpeng362@huawei.com>
https://mailweb.openeuler.org/hyperkitty/list/kernel@openeuler.org/message/26G2BQVLWGLNTTWKVCSAIOGTT4BVILT4/ 
 
https://gitee.com/src-openeuler/kernel/issues/I96GP1 
 
Link:https://gitee.com/openeuler/kernel/pulls/5220

 

Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Reviewed-by: Liu YongQiang <liuyongqiang13@huawei.com>
Signed-off-by: Zhang Changzhong <zhangchangzhong@huawei.com>