ocfs2: don't use flush_scheduled_work()

flush_scheduled_work() is deprecated and scheduled to be removed.

* cancel_delayed_work() + flush_schedule_work() ->
  cancel_delayed_work_sync().

* flush qs->qs_work directly on exit instead.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Joel Becker <joel.becker@oracle.com>
Cc: Mark Fasheh <mfasheh@suse.com>