ip_tunnel: fix potential issue in ip_tunnel_rcv

When ip_tunnel_rcv fails, the tun_dst won't be freed, so call
dst_release to free it in error code path.

Fixes: 2e15ea39 ("ip_gre: Add support to collect tunnel metadata.")
Acked-by: NEric Dumazet <edumazet@google.com>
Acked-by: NPravin B Shelar <pshelar@ovn.org>
Tested-by: NZhang Shengju <zhangshengju@cmss.chinamobile.com>
Signed-off-by: NHaishuang Yan <yanhaishuang@cmss.chinamobile.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/ip_tunnel.c

@@ -446,6 +446,8 @@ int ip_tunnel_rcv(struct ip_tunnel *tunnel, struct sk_buff *skb,
 	return 0;
 
 drop:
+	if (tun_dst)
+		dst_release((struct dst_entry *)tun_dst);
 	kfree_skb(skb);
 	return 0;
 }