net: original ingress device index in PKTINFO

Applications such as OSPF and BFD need the original ingress device not
the VRF device; the latter can be derived from the former. To that end
add the skb_iif to inet_skb_parm and set it in ipv4 code after clearing
the skb control buffer similar to IPv6. From there the pktinfo can just
pull it from cb with the PKTINFO_SKB_CB cast.

The previous patch moving the skb->dev change to L3 means nothing else
is needed for IPv6; it just works.

Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/ip_sockglue.c b/net/ipv4/ip_sockglue.c
index bdb222c..5805762 100644
--- a/net/ipv4/ip_sockglue.c
+++ b/net/ipv4/ip_sockglue.c
@@ -1193,7 +1193,12 @@
 		       ipv6_sk_rxinfo(sk);
 
 	if (prepare && skb_rtable(skb)) {
-		pktinfo->ipi_ifindex = inet_iif(skb);
+		/* skb->cb is overloaded: prior to this point it is IP{6}CB
+		 * which has interface index (iif) as the first member of the
+		 * underlying inet{6}_skb_parm struct. This code then overlays
+		 * PKTINFO_SKB_CB and in_pktinfo also has iif as the first
+		 * element so the iif is picked up from the prior IPCB
+		 */
 		pktinfo->ipi_spec_dst.s_addr = fib_compute_spec_dst(skb);
 	} else {
 		pktinfo->ipi_ifindex = 0;