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: Artem Savkov <artem.savkov@gmail.com>