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: Eric Dumazet <edumazet@google.com>
Acked-by: Pravin B Shelar <pshelar@ovn.org>
Tested-by: Zhang Shengju <zhangshengju@cmss.chinamobile.com>
Signed-off-by: Haishuang Yan <yanhaishuang@cmss.chinamobile.com>
Signed-off-by: David S. Miller <davem@davemloft.net>