ipv6: drop metadata dst in ip6_route_input

The fix in commit 48fb6b55 is incomplete, as now ip6_route_input can be
called with non-NULL dst if it's a metadata dst and the reference is leaked.
Drop the reference.

Fixes: 48fb6b55 ("ipv6: fix crash over flow-based vxlan device")
Fixes: ee122c79 ("vxlan: Flow based tunneling")
CC: Wei-Chun Chao <weichunc@plumgrid.com>
CC: Thomas Graf <tgraf@suug.ch>
Signed-off-by: NJiri Benc <jbenc@redhat.com>
Acked-by: NThomas Graf <tgraf@suug.ch>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/route.c

@@ -1140,6 +1140,7 @@ void ip6_route_input(struct sk_buff *skb)
 		.flowi6_proto = iph->nexthdr,
 	};
 
+	skb_dst_drop(skb);
 	skb_dst_set(skb, ip6_route_input_lookup(net, skb->dev, &fl6, flags));
 }