fix use_dest_devs for rebalance

The caller expects an array ref, currently using use_dest_devs will kill
JobMaster with:

Oct 18 09:45:25 storage22 mogilefsd[23263]: crash log: rebalance cannot find suitable destination devices at /usr/local/share/perl/5.10.1/MogileFS/Worker/JobMaster.pm line 233
Oct 18 09:45:26 storage22 mogilefsd[22044]: Child 23263 (job_master) died: 256 (UNEXPECTED)