Item vfs.fs.size first parameter is expecting mount point but not the filesystem device.

Examle host:

1. df -h
   Filesystem Size Used Avail Use% Mounted on
   /dev/root 9.9G 7.4G 2.4G 76% /
   devtmpfs 12G 0 12G 0% /dev
   run 12G 1.8M 12G 1% /run
   shm 12G 0 12G 0% /dev/shm
   /dev/drbd111 1.9T 1.9T 78G 96% /matrix/v025

1. zabbix_agentd -c /etc/zabbix.conf -t vfs.fs.size[/matrix/v025,pused]
   vfs.fs.size[/matrix/v025,pused] [d|95.954955]

1. zabbix_agentd -c /etc/zabbix.conf -t vfs.fs.size[/dev/drbd111,pused]
   vfs.fs.size[/dev/drbd111,pused] [d|0.000000]