LU-8740 lfsck: hold lock when access trace file object

LU-8740 lfsck: hold lock when access trace file object

There is race condition between lfsck_in_notify() access the trace
file object and the lfsck_namespace_load_sub_trace_files() that may
re-create the trace file. Hold lfsck_sub_trace_obj::lsto_mutex and
check check the validaty of the trace file object to avoid trouble.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I9715524dd7027f5fc8c7078c1a52d099e9e21132
Reviewed-on: https://review.whamcloud.com/23301
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>