MariaDB does not start after restoring incremental backups with mariabackup
When the permissions look correct, but you're still getting Errcode: 13 "Permission denied", then the next thing to check is whether a security system is causing the problem. On CentOS that system is SELinux. To check if it's running in enforcing mode, execute this: 

sestatus


You can do setenforce 0 to temporarily change it to permissive mode.

To avoid the problem:

When you initially install MariaDB, the SELinux security context should be set correctly for all the files. However, when you do mv /var/lib/mysql/ /var/lib/mysql_bak/, then you remove it. I think you can get it back by simply executing restorecon -Rv /var/lib/mysql.

For reference, read this page in the MariaDB Knowledgebase: Security-Enhanced Linux with MariaDB