macsec: don't free NULL metadata_dst

Commit 0a28bfd4 added a metadata_dst to each tx_sc, but that's
only allocated when macsec_add_dev has run, which happens after device
registration. If the requested or computed SCI already exists, or if
linking to the lower device fails, we will panic because
metadata_dst_free can't handle NULL.

Reproducer:
    ip link add link $lower type macsec
    ip link add link $lower type macsec

Fixes: 0a28bfd4 ("net/macsec: Add MACsec skb_metadata_dst Tx Data path support")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Acked-by: Raed Salem <raeds@nvidia.com>
Link: https://lore.kernel.org/r/60f2a1965fe553e2cade9472407d0fafff8de8ce.1663923580.git.sd@queasysnail.netSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/macsec.c

@@ -3720,7 +3720,8 @@ static void macsec_free_netdev(struct net_device *dev)
 {
 	struct macsec_dev *macsec = macsec_priv(dev);
 
-	metadata_dst_free(macsec->secy.tx_sc.md_dst);
+	if (macsec->secy.tx_sc.md_dst)
+		metadata_dst_free(macsec->secy.tx_sc.md_dst);
 	free_percpu(macsec->stats);
 	free_percpu(macsec->secy.tx_sc.stats);