LU-9294 mgs: ignore stale barrier locks

LU-9294 mgs: ignore stale barrier locks

Currently, when MDT umount/crash, it may not notify the MGS
to cleanup related barrier lock and bitmap. Then subsequent
barrier operation may find some stale barrier locks. It is
unnecessary to return failure by mgs_barrier_glimpse_lock()
for such case, the caller will handle that properly.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I341fd648da2eebbaab729145d1f06c420fce6455
Reviewed-on: https://review.whamcloud.com/26455
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>