vti6: fix input path

Since commit 1625f4529957, vti6 is broken, all input packets are dropped
(LINUX_MIB_XFRMINNOSTATES is incremented).

XFRM_TUNNEL_SKB_CB(skb)->tunnel.ip6 is set by vti6_rcv() before calling
xfrm6_rcv()/xfrm6_rcv_spi(), thus we cannot set to NULL that value in
xfrm6_rcv_spi().

A new function xfrm6_rcv_tnl() that enables to pass a value to
xfrm6_rcv_spi() is added, so that xfrm6_rcv() is not touched (this function
is used in several handlers).

CC: Alexey Kodanev <alexey.kodanev@oracle.com>
Fixes: 1625f4529957 ("net/xfrm_input: fix possible NULL deref of tunnel.ip6->parms.i_key")
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
diff --git a/net/ipv6/xfrm6_tunnel.c b/net/ipv6/xfrm6_tunnel.c
index 5743044..e1c0bbe 100644
--- a/net/ipv6/xfrm6_tunnel.c
+++ b/net/ipv6/xfrm6_tunnel.c
@@ -236,7 +236,7 @@
 	__be32 spi;
 
 	spi = xfrm6_tunnel_spi_lookup(net, (const xfrm_address_t *)&iph->saddr);
-	return xfrm6_rcv_spi(skb, IPPROTO_IPV6, spi);
+	return xfrm6_rcv_spi(skb, IPPROTO_IPV6, spi, NULL);
 }
 
 static int xfrm6_tunnel_err(struct sk_buff *skb, struct inet6_skb_parm *opt,