vhost: wake up worker outside spin_lock

the wake_up_process func is included by spin_lock/unlock in
vhost_work_queue,
but it could be done outside the spin_lock.
I have test it with kernel 3.0.27 and guest suse11-sp2 using iperf,
the num as below.
                  original                 modified
thread_num  tp(Gbps)   vhost(%)  |  tp(Gbps)     vhost(%)
1           9.59        28.82    |   9.59        27.49
8           9.61        32.92    |   9.62        26.77
64          9.58        46.48    |   9.55        38.99
256         9.6         63.7     |   9.6         52.59
Signed-off-by: NChuanyu Qin <qinchuanyu@huawei.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

drivers/vhost/vhost.c

@@ -161,9 +161,11 @@ void vhost_work_queue(struct vhost_dev *dev, struct vhost_work *work)
 	if (list_empty(&work->node)) {
 		list_add_tail(&work->node, &dev->work_list);
 		work->queue_seq++;
+		spin_unlock_irqrestore(&dev->work_lock, flags);
 		wake_up_process(dev->worker);
+	} else {
+		spin_unlock_irqrestore(&dev->work_lock, flags);
 	}
-	spin_unlock_irqrestore(&dev->work_lock, flags);
 }
 EXPORT_SYMBOL_GPL(vhost_work_queue);