LU-12974 lfsck: dryrun mode shouldn't repair for layout

LU-12974 lfsck: dryrun mode shouldn't repair for layout

"lfsck_start -t layout --dryrun" shouldn't repair but report
inconsistencies.

Update sanity-lfsck.sh 18b.

Test-parameters: trivial testlist=sanity-lfsck envdefinitions=ONLY=18
DDN-Bug-ID: DDN-892

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: Ie1f4d0d1726e8f3f39f5a8b7be9cc149df1c1e44
Reviewed-on: https://review.whamcloud.com/36767
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Hongchao Zhang <hongchao@whamcloud.com>