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: I9b067f50df81461702c7f3db0eadd2bf3ebf5699
Reviewed-on: http://review.whamcloud.com/4252
Tested-by: Hudson
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Bobi Jam <bobijam@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>