LU-12068 test: compare position for ZFS dot entry

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.

Lustre-change: https://review.whamcloud.com/34525
Lustre-commit: 42adbae36f206a6ed4170e7619cd993c8fa80b1d

Change-Id: I98aee1ae92fa5ea742a8001b58e092111d646477
Signed-off-by: Hongchao Zhang <hongchao@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Signed-off-by: Minh Diep <mdiep@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/34696
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>