hv_netvsc: Add comment of netvsc_xdp_xmit()

Adding comment to avoid the misusing of netvsc_xdp_xmit().
Otherwise the value of skb->queue_mapping could be 0 and
then the return value of skb_get_rx_queue() could be MAX_U16
cause by overflow.

Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Reviewed-by: Haiyang Zhang <haiyangz@microsoft.com>
Link: https://lore.kernel.org/r/1634174786-1810351-1-git-send-email-jiasheng@iscas.ac.cn
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/hyperv/netvsc_drv.c

@@ -803,6 +803,7 @@ void netvsc_linkstatus_callback(struct net_device *net,
 	schedule_delayed_work(&ndev_ctx->dwork, 0);
 }
 
+/* This function should only be called after skb_record_rx_queue() */
 static void netvsc_xdp_xmit(struct sk_buff *skb, struct net_device *ndev)
 {
 	int rc;