LU-9040 scrub: detect dead loop scanning

LU-9040 scrub: detect dead loop scanning

It is found that the OI scrub may fall into dead loop scanning
for some unknown reason. This patch checks the scanning cursor
to make sure it will not scan the same object repeatedly.

It also fixes a logic error about 'noscrub' handling, that may
cause the OI scrub to fall into dead loop scanning when the OI
scrub resumes from former crashed partial scanning.

Test-Parameters: mdtfilesystemtype=ldiskfs ostfilesystemtype=ldiskfs mdscount=2 mdtcount=4 testlist=sanity-scrub,sanity-scrub,sanity-scrub
Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: Ia1f63e8a2d675e9fa4567fa329905ac769b83a74
Reviewed-on: https://review.whamcloud.com/26751
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>