LU-2142 scrub: reset completed scrub position if retrigger

LU-2142 scrub: reset completed scrub position if retrigger

If former OI scrub has been completed, and the user wants to run
the OI scrub again, then reset the OI scrub to make it to rescan
the device from the beginning.

Signed-off-by: Fan Yong <yong.fan@whamcloud.com>
Change-Id: I5b8e9ee51ccbf95ed131b963389c4ecfb92b9035
Reviewed-on: http://review.whamcloud.com/4250
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Bobi Jam <bobijam@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>