LU-9725 lwp: wait on deregister

LU-9725 lwp: wait on deregister

When lustre_deregister_lwp_item() is being called, it should wait
for the inflight callback done before moving on to free the data
used by the callback.

Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: I9c27a0ae4c765147fd183b78bf3693a66e7511dc
Reviewed-on: https://review.whamcloud.com/27987
Tested-by: Jenkins
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>