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/ip6_vti.c b/net/ipv6/ip6_vti.c
index 52a2f73..5bd3afd 100644
--- a/net/ipv6/ip6_vti.c
+++ b/net/ipv6/ip6_vti.c
@@ -321,11 +321,9 @@
 			goto discard;
 		}
 
-		XFRM_TUNNEL_SKB_CB(skb)->tunnel.ip6 = t;
-
 		rcu_read_unlock();
 
-		return xfrm6_rcv(skb);
+		return xfrm6_rcv_tnl(skb, t);
 	}
 	rcu_read_unlock();
 	return -EINVAL;