!8357 nilfs2: fix underflow in second superblock position calculations

Merge Pull Request from: @ci-robot 
 
PR sync from: Zizhi Wo <wozizhi@huawei.com>
https://mailweb.openeuler.org/hyperkitty/list/kernel@openeuler.org/message/YA53CSX35ID7FNKP73GTT6OUDOC4IS37/ 
 
https://gitee.com/src-openeuler/kernel/issues/I9R4KZ 
 
Link:https://gitee.com/openeuler/kernel/pulls/8357

 

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