LU-7861 osp: replace the hot spinlock with atomic trackers

LU-7861 osp: replace the hot spinlock with atomic trackers

so that background scanning doesn't race with MDS_REINT_UNLINK

Change-Id: I4006be4298ffa841ca4d131ac37c201ff8ea729f
Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-on: http://review.whamcloud.com/19211
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>