LU-5264 obdclass: fix race during key quiescency

LU-5264 obdclass: fix race during key quiescency

Upon umount, presumably of last device using same OSD back-end,
to prepare for module unload, lu_context_key_quiesce() is run to
remove all module's key reference in any context linked on
lu_context_remembered list.
Threads must protect against such transversal processing when
exiting from its context.

Signed-off-by: Bruno Faccini <bruno.faccini@intel.com>
Change-Id: If2c8199fa764236308b49950672129a63b8877f5
Reviewed-on: http://review.whamcloud.com/13103
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>