Quotas will fail on CentOS 6 on your linode as by default it will be looking for /dev/root. You can fix the issue by adding a symbolic link. The below will work for DirectAdmin, cPanel and Virtualmin.
ln -s /dev/xvda /dev/root
Restarting your server will lose the link so you can stick the following in the /etc/rc.sysinit file just above the comment "# Update quotas if necessary"
# Create link to /dev/root for quota fix ln -s /dev/xvda /dev/root