ip6_gre: fix a dst leak in ip6erspan_tunnel_xmit

In ip6erspan_tunnel_xmit(), if the skb will not be sent out, it has to
be freed on the tx_err path. Otherwise when deleting a netns, it would
cause dst/dev to leak, and dmesg shows:

  unregister_netdevice: waiting for lo to become free. Usage count = 1

Fixes: ef7baf5e ("ip6_gre: add ip6 erspan collect_md mode")
Signed-off-by: NXin Long <lucien.xin@gmail.com>
Acked-by: NWilliam Tu <u9012063@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/ip6_gre.c

@@ -968,7 +968,7 @@ static netdev_tx_t ip6erspan_tunnel_xmit(struct sk_buff *skb,
 		if (unlikely(!tun_info ||
 			     !(tun_info->mode & IP_TUNNEL_INFO_TX) ||
 			     ip_tunnel_info_af(tun_info) != AF_INET6))
-			return -EINVAL;
+			goto tx_err;
 
 		key = &tun_info->key;
 		memset(&fl6, 0, sizeof(fl6));