LU-5788 test: fix the cmd to getfree space

LU-5788 test: fix the cmd to getfree space

The cmd to get free space in run_tar.sh and run_dd.sh is obsolete
after the patch https://review.whamcloud.com/24228 was landed.

Test-Parameters: trivial testlist=recovery-double-scale clientdistro=el7 serverdistro=el7 \
osscount=2 mdscount=2 ostcount=7 mdtcount=1 clientcount=4 ostfilesystemtype=ldiskfs mdtfilesystemtype=ldiskfs failover=true iscsi=1

Change-Id: I442825a94102eff68aaba411b57f86d65441ae49
Signed-off-by: Hongchao Zhang <hongchao.zhang@intel.com>
Reviewed-on: https://review.whamcloud.com/26031
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Nunez <james.a.nunez@intel.com>
Reviewed-by: Wei Liu <wei3.liu@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>