LU-3032 ptlrpc: race in pinger (use-after-free situation)

The race is result of use-after-free situation:

~ ptlrpc_stop_pinger()          ~ ptlrpc_pinger_main()
---------------------------------------------------------------
thread_set_flags(SVC_STOPPING)
cfs_waitq_signal(pinger_thread) ...
...                             thread_set_flags(SVC_STOPPED)
l_wait_event(thread_is_stopped)
OBD_FREE_PTR(pinger_thread)
...                             cfs_waitq_signal(pinger_thread)
---------------------------------------------------------------

The memory used by pinger_thread might have been freed and
reallocated to something else, when ptlrpc_pinger_main()
used it in cvs_waitq_signal().

Signed-off-by: Li Wei <wei.g.li@intel.com>
Signed-off-by: Dmitry Eremin <dmitry.eremin@intel.com>
Change-Id: I3516c7ece2b2e1e4e0b2e47acb0583c174a3e631
Reviewed-on: http://review.whamcloud.com/6040
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Faccini Bruno <bruno.faccini@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>