LU-8569 lfsck: cleanup lfsck requests list before exit

LU-8569 lfsck: cleanup lfsck requests list before exit

When the lfsck assistant thread hits some failure and exit
at the second stage scanning, it does not cleanup the requests
list 'lfsck_assistant_data::lad_req_list', that may cause the
lfsck main engine hit "LASSERT(list_empty(&lad->lad_req_list))"
when handles double scan.

This patch unifies the assistant thread exit process: before
cleanup the list 'lfsck_assistant_data::lad_req_list' set the
thread as "stopping" to prevent more lfsck requests being added,
then cleanup the list 'lfsck_assistant_data::lad_req_list'.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I7facd20e2742c7ad5d4fbee1c975dacd8f6ea363
Reviewed-on: http://review.whamcloud.com/22723
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@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>