LU-12068 test: compare position for ZFS dot entry

in test_6b of sanity-lfsck.sh, the position will be zero for
special entries "." and "..", which should not be used to
determine whether the LFSCK process is forward or not, in this
case, the otable position should be used.

Change-Id: I98aee1ae92fa5ea742a8001b58e092111d646477
Signed-off-by: Hongchao Zhang <hongchao@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/34525
Tested-by: Jenkins
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>