Proxmox 2.1 backup via web interface

this flag is removed.

instead, you can set now the maxfiles option on the storage definition.
 
I change the option in storage but the script in /etc/cron.d/vzdump is no altered by this option.

Is it normal ?
 
yes. you can remove the maxfiles option from the /etc/cron.d/vzdump, its handled on the storage definition.

see /etc/pve/storage.cfg