ipv4: Fix "Set rt->rt_iif more sanely on output routes."

Commit 1018b5c01636c7c6bda31a719bda34fc631db29a ("Set rt->rt_iif more
sanely on output routes.")  breaks rt_is_{output,input}_route.

This became the cause to return "IP_PKTINFO's ->ipi_ifindex == 0".

To fix it, this does:

1) Add "int rt_route_iif;" to struct rtable

2) For input routes, always set rt_route_iif to same value as rt_iif

3) For output routes, always set rt_route_iif to zero.  Set rt_iif
   as it is done currently.

4) Change rt_is_{output,input}_route() to test rt_route_iif

Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/xfrm4_policy.c b/net/ipv4/xfrm4_policy.c
index 13e0e7f..d20a05e 100644
--- a/net/ipv4/xfrm4_policy.c
+++ b/net/ipv4/xfrm4_policy.c
@@ -74,6 +74,7 @@
 	rt->rt_key_dst = fl4->daddr;
 	rt->rt_key_src = fl4->saddr;
 	rt->rt_tos = fl4->flowi4_tos;
+	rt->rt_route_iif = fl4->flowi4_iif;
 	rt->rt_iif = fl4->flowi4_iif;
 	rt->rt_oif = fl4->flowi4_oif;
 	rt->rt_mark = fl4->flowi4_mark;