After the upgrade { 1) stop mariadb service, 2) yum remove and 3) yum install mariadb-server }, I executed the script mysql_upgrade and I go the unix_socket not loaded error.
# mysql_upgrade
Version check failed. Got the following error when calling the 'mysql' command line client
ERROR 1524 (HY000): Plugin 'unix_socket' is not loaded
FATAL ERROR: Upgrade failed
Later, I tried to mysqld-safe --skip-grant-tables and logged into MariaDB, noticed that root user for localhost contains plugin as unix_socket.