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>