ipv6/ip6_gre: fix error case handling in ip6gre_tunnel_xmit()

ip6gre_tunnel_xmit() is leaking the skb when we hit this error branch,
and the -1 return value from this function is bogus. Use the error
handling we already have in place in ip6gre_tunnel_xmit() for this error
case to fix this.
Signed-off-by: NTommi Rantala <tt.rantala@gmail.com>
Acked-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/ip6_gre.c

@@ -960,7 +960,7 @@ static netdev_tx_t ip6gre_tunnel_xmit(struct sk_buff *skb,
 	int ret;
 
 	if (!ip6_tnl_xmit_ctl(t))
-		return -1;
+		goto tx_err;
 
 	switch (skb->protocol) {
 	case htons(ETH_P_IP):