[PATCH net-next] ip_tunnel:multicast process cause panic due to skb->_skb_refdst NULL pointer

when ip_tunnel process multicast packets, it may check if the packet is
looped back packet though 'rt_is_output_route(skb_rtable(skb))' in
ip_tunnel_rcv(), but before that , skb->_skb_refdst has been dropped in
iptunnel_pull_header(), so which leads to a panic.

fix the bug: https://bugzilla.kernel.org/show_bug.cgi?id=70681

diff --git a/net/ipv4/ip_tunnel_core.c b/net/ipv4/ip_tunnel_core.c
index 6156f4e..88b08aa 100644
--- a/net/ipv4/ip_tunnel_core.c
+++ b/net/ipv4/ip_tunnel_core.c

@@ -108,7 +108,6 @@ int iptunnel_pull_header(struct sk_buff *skb, int 

hdr_len, __be16 inner_proto)
         nf_reset(skb);
         secpath_reset(skb);
         skb_clear_hash_if_not_l4(skb);
-       skb_dst_drop(skb);
         skb->vlan_tci = 0;
         skb_set_queue_mapping(skb, 0);
         skb->pkt_type = PACKET_HOST;