Returns the total amount, in bytes, of unused storage
space currently available to this file system.
Additional information:
This objects is only supported if a disk is
installed.
Returns <noSuchName> status if
attempting to perform a Get on this OID with no disk
installed.
Returns <noSuchName>
status if attempting to perform a Set, EnableTrap, or
DisableTrap on this OID with no disk installed.