mariadb.service: Start operation timed out.

无法启动mariadb服务，查看mariadb服务状态如下： 

解决方法：

1. echo "/usr/sbin/mysqld { }" > /etc/apparmor.d/usr.sbin.mysqld
2. apparmor_parser -v -R /etc/apparmor.d/usr.sbin.mysqld
3. systemctl restart mariadb

参考：https://stackoverflow.com/questions/40997257/mysql-service-fails-to-start-hangs-up-timeout-ubuntu-mariadb