LU-9140 nrs: measure the runtime of dd directly

LU-9140 nrs: measure the runtime of dd directly

This patch changes the way to measure the runtime of "dd". Instead
of parsing the output of "dd", we use date command to calculate
the runtime of dd directly, avoiding the parsing failure caused
by changed output format of "dd".

Lustre-change: https://review.whamcloud.com/27878
Lustre-commit: 66abf6ffe9f032aaa7c271ee2bd1f9b569058bd5

Change-Id: Ibd2e3963f791404ee927981238227012cf4dbf2c
Test-Parameters: trivial testlist=sanityn
Signed-off-by: Qian Yingjin <qian@ddn.com>
Reviewed-by: James Nunez <james.a.nunez@intel.com>
Signed-off-by: Minh Diep <minh.diep@intel.com>
Reviewed-on: https://review.whamcloud.com/29686
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>