sit: fix 4in4 + IPsec scenario

Since commit 32b8a8e5 "sit: add IPv4 over IPv4 support",
tunnel->parms.iph.protocol is 0 when both 4in4 and 6in4 are setup, but
xfrm_lookup() is called only when proto is != 0, thus we need to pass the real
value.
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_tunnel.c

@@ -542,7 +542,7 @@ void ip_tunnel_xmit(struct sk_buff *skb, struct net_device *dev,
 	}
 
 	rt = ip_route_output_tunnel(dev_net(dev), &fl4,
-				    tunnel->parms.iph.protocol,
+				    protocol,
 				    dst, tnl_params->saddr,
 				    tunnel->parms.o_key,
 				    RT_TOS(tos),