ALSA: usb-audio: Fix racy list management in output queue

The linked list entry from FIFO is peeked at
queue_pending_output_urbs() but the actual element pop-out is
performed outside the spinlock, and it's potentially racy.

Do delete the link at the right place inside the spinlock.

Fixes: 8fdff6a3 ("ALSA: snd-usb: implement new endpoint streaming model")
Link: https://lore.kernel.org/r/20200424074016.14301-1-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/endpoint.c

@@ -346,17 +346,17 @@ static void queue_pending_output_urbs(struct snd_usb_endpoint *ep)
 			ep->next_packet_read_pos %= MAX_URBS;
 
 			/* take URB out of FIFO */
-			if (!list_empty(&ep->ready_playback_urbs))
+			if (!list_empty(&ep->ready_playback_urbs)) {
 				ctx = list_first_entry(&ep->ready_playback_urbs,
 					       struct snd_urb_ctx, ready_list);
+				list_del_init(&ctx->ready_list);
+			}
 		}
 		spin_unlock_irqrestore(&ep->lock, flags);
 
 		if (ctx == NULL)
 			return;
 
-		list_del_init(&ctx->ready_list);
-
 		/* copy over the length information */
 		for (i = 0; i < packet->packets; i++)
 			ctx->packet_size[i] = packet->packet_size[i];