macsec: set actual real device for xmit when !protect_frames

Avoid recursions of dev_queue_xmit() to the wrong net device when
frames are unprotected, since at that time skb->dev still points to
our own macsec dev and unlike macsec_encrypt_finish() dev pointer
doesn't get updated to real underlying device.

Fixes: c09440f7 ("macsec: introduce IEEE 802.1AE driver")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Sabrina Dubroca <sd@queasysnail.net>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/macsec.c

@@ -2640,6 +2640,7 @@ static netdev_tx_t macsec_start_xmit(struct sk_buff *skb,
 		u64_stats_update_begin(&secy_stats->syncp);
 		secy_stats->stats.OutPktsUntagged++;
 		u64_stats_update_end(&secy_stats->syncp);
+		skb->dev = macsec->real_dev;
 		len = skb->len;
 		ret = dev_queue_xmit(skb);
 		count_tx(dev, ret, len);