rcutorture: NULL rcu_torture_current earlier in cleanup code

Currently, the rcu_torture_current variable remains non-NULL until after
all readers have stopped.  During this time, rcu_torture_stats_print()
will think that the test is still ongoing, which can result in confusing
dmesg output.  This commit therefore NULLs rcu_torture_current immediately
after the rcu_torture_writer() kthread has decided to stop, thus informing
rcu_torture_stats_print() much sooner.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/rcutorture.c

@@ -1172,6 +1172,7 @@ rcu_torture_writer(void *arg)
 					WARN(1, "%s: rtort_pipe_count: %d\n", __func__, rcu_tortures[i].rtort_pipe_count);
 				}
 	} while (!torture_must_stop());
+	rcu_torture_current = NULL;  // Let stats task know that we are done.
 	/* Reset expediting back to unexpedited. */
 	if (expediting > 0)
 		expediting = -expediting;
@@ -2473,7 +2474,6 @@ rcu_torture_cleanup(void)
 					     reader_tasks[i]);
 		kfree(reader_tasks);
 	}
-	rcu_torture_current = NULL;
 
 	if (fakewriter_tasks) {
 		for (i = 0; i < nfakewriters; i++) {