LU-657 test: limit the write size in run_dd

LU-657 test: limit the write size in run_dd

1, in run_dd.sh, limit the write size of dd according to
   the free disk space currently intead of using fixed value.

2, fix a bug in ofd_tnx_stop_cb, which cleared the 'thandle->th_sync'

Test-Parameters: envdefinitions=SLOW=yes clientcount=4 osscount=2 mdscount=2 austeroptions=-R failover=true useiscsi=true testlist=recovery-mds-scale
Signed-off-by: Hongchao Zhang <hongchao.zhang@whamcloud.com>
Change-Id: I8b9cc2fc8b9831e5ad79b521ba7af4ae9160143f
Reviewed-on: http://review.whamcloud.com/4599
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>