ipv6: fix crash over flow-based vxlan device

Similar check was added in ip_rcv but not in ipv6_rcv.

BUG: unable to handle kernel NULL pointer dereference at (null)
IP: [<ffffffff81734e0a>] ipv6_rcv+0xfa/0x500
Call Trace:
[<ffffffff816c9786>] ? ip_rcv+0x296/0x400
[<ffffffff817732d2>] ? packet_rcv+0x52/0x410
[<ffffffff8168e99f>] __netif_receive_skb_core+0x63f/0x9a0
[<ffffffffc02b34a0>] ? br_handle_frame_finish+0x580/0x580 [bridge]
[<ffffffff8109912c>] ? update_rq_clock.part.81+0x1c/0x40
[<ffffffff8168ed18>] __netif_receive_skb+0x18/0x60
[<ffffffff8168fa1f>] process_backlog+0x9f/0x150

Fixes: ee122c79d422 (vxlan: Flow based tunneling)
Signed-off-by: Wei-Chun Chao <[e-mail]>
Acked-by: Thomas Graf <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/ipv6/ip6_input.c

···
 #include <net/addrconf.h>
 #include <net/xfrm.h>
 #include <net/inet_ecn.h>
+#include <net/dst_metadata.h>
 
 int ip6_rcv_finish(struct sock *sk, struct sk_buff *skb)
 {
···
 		if (ipprot && ipprot->early_demux)
 			ipprot->early_demux(skb);
 	}
-	if (!skb_dst(skb))
+	if (!skb_valid_dst(skb))
 		ip6_route_input(skb);
 
 	return dst_input(skb);
···
 	 * arrived via the sending interface (ethX), because of the
 	 * nature of scoping architecture. --yoshfuji
 	 */
-	IP6CB(skb)->iif = skb_dst(skb) ? ip6_dst_idev(skb_dst(skb))->dev->ifindex : dev->ifindex;
+	IP6CB(skb)->iif = skb_valid_dst(skb) ? ip6_dst_idev(skb_dst(skb))->dev->ifindex : dev->ifindex;
 
 	if (unlikely(!pskb_may_pull(skb, sizeof(*hdr))))
 		goto err;