LU-5624 tests: ignore bad lfsck performance for ZFS backend

Currently, the LFSCK performance has only been verified against
ldiskfs-based backend. The ZFS-based backend may hit bad performance
in sanity-lfsck test_9. Ignore the slow performance for ZFS-based
backend until we resolved the ZFS performance issues.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: Ic70e33b1e10c1798b3df01039c34ac8f00d14375
Reviewed-on: http://review.whamcloud.com/12322
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>