rcuperf: Do not wake up shutdown wait queue if "shutdown" is false.

After finishing its tests rcuperf tries to wake up shutdown_wq even if
"shutdown" param is set to false, resulting in a wake_up() call on an
unitialized wait_queue_head_t which leads to "BUG: spinlock bad magic" and
"BUG: unable to handle kernel NULL pointer dereference".

Fix by checking "shutdown" param before waking up the queue.
Signed-off-by: NArtem Savkov <artem.savkov@gmail.com>

kernel/rcu/rcuperf.c

@@ -423,10 +423,12 @@ rcu_perf_writer(void *arg)
 					b_rcu_perf_writer_finished =
 						cur_ops->completed();
 				}
+				if (shutdown) {
 					smp_mb(); /* Assign before wake. */
 					wake_up(&shutdown_wq);
 				}
 			}
+		}
 		if (done && !alldone &&
 		    atomic_read(&n_rcu_perf_writer_finished) >= nrealwriters)
 			alldone = true;