LU-9820 osd-ldiskfs: OI scrub speed limit fix

LU-9820 osd-ldiskfs: OI scrub speed limit fix

The OI scrub speed limit is set at start time, and shouldn't be
changed in the middle, otherwise lfsck is run with speed control,
while OI scrub may finish before it, thus sanity-scrub 9 failed.

Allow wider margin in speed control since the speed control is
heuristic.

Test-Parameters: testlist=sanity-scrub,sanity-scrub,sanity-scrub,sanity-scrub,sanity-scrub,sanity-scrub,sanity-scrub,sanity-scrub,sanity-scrub,sanity-scrub,sanity-scrub,sanity-scrub,sanity-scrub
Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: I995679db085a193c71fc7914f961b538b5930c69
Reviewed-on: https://review.whamcloud.com/40591
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Hongchao Zhang <hongchao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>