LU-10615 osd: stop OI scrub before FLDB closed

LU-10615 osd: stop OI scrub before FLDB closed

OI scrub may check FLDB when scans the device. During umount
the device, we need to stop OI scrub before closing the FLDB
to void invalid RAM accessing.

Some code optimization and cleanup.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: Ib358abd77f970c12b0c29a603f9bcaf8e310cc98
Reviewed-on: https://review.whamcloud.com/31241
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>