LU-9140 nrs: calculate the measured rate according to dd runtime

LU-9140 nrs: calculate the measured rate according to dd runtime

This patch help to calculate the measured IOPS by dd according
to dd runtime, instead of obtaining the result from the
performance data of dd output which value changes according to
its unit: 'MB/s', 'GB/s' or 'kB/s'.

Change-Id: Iace926d5c001f20b1c1d89ad6124d14a80316e86
Test-Parameters: trivial testlist=sanityn,sanityn,sanity
Signed-off-by: Qian Yingjin <qian@ddn.com>
Reviewed-on: https://review.whamcloud.com/27110
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Wang Shilong <wshilong@ddn.com>