usb: gadget: function: fix missing spinlock in f_uac1_legacy

Add a missing spinlock protection for play_queue, because
the play_queue may be destroyed when the "playback_work"
work func and "f_audio_out_ep_complete" callback func
operate this paly_queue at the same time.

Fixes: c6994e6f ("USB: gadget: add USB Audio Gadget driver")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Zhang Qiang <qiang.zhang@windriver.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>

drivers/usb/gadget/function/f_uac1_legacy.c

@@ -336,7 +336,9 @@ static int f_audio_out_ep_complete(struct usb_ep *ep, struct usb_request *req)
 
 	/* Copy buffer is full, add it to the play_queue */
 	if (audio_buf_size - copy_buf->actual < req->actual) {
+		spin_lock_irq(&audio->lock);
 		list_add_tail(&copy_buf->list, &audio->play_queue);
+		spin_unlock_irq(&audio->lock);
 		schedule_work(&audio->playback_work);
 		copy_buf = f_audio_buffer_alloc(audio_buf_size);
 		if (IS_ERR(copy_buf))