ip6_gre: release cached dst on tunnel removal

commit d4257295 upstream.

When a tunnel is deleted, the cached dst entry should be released.

This problem may prevent the removal of a netns (seen with a x-netns IPv6
gre tunnel):
  unregister_netdevice: waiting for lo to become free. Usage count = 3

CC: Dmitry Kozlov <xeb@mail.ru>
Fixes: c12b395a ("gre: Support GRE over IPv6")
Signed-off-by: huaibin Wang <huaibin.wang@6wind.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[ kamal: backport to 3.13-stable ]
Signed-off-by: Kamal Mostafa <kamal@canonical.com>

net/ipv6/ip6_gre.c

@@ -360,6 +360,7 @@ static void ip6gre_tunnel_uninit(struct net_device *dev)
 	struct ip6gre_net *ign = net_generic(net, ip6gre_net_id);
 
 	ip6gre_tunnel_unlink(ign, netdev_priv(dev));
+	ip6_tnl_dst_reset(netdev_priv(dev));
 	dev_put(dev);
 }