ipmr: support IP_PKTINFO on cache report IGMP msg

In order to support IP_PKTINFO on those packets, we need to call
ipv4_pktinfo_prepare.

When sending mrouted/pimd daemons a cache report IGMP msg, it is
unnecessary to set dst on the newly created skb.
It used to be necessary on older versions until
commit d826eb1 ("ipv4: PKTINFO doesnt need dst reference") which
changed the way IP_PKTINFO struct is been retrieved.

Changes from v1:
1. Undo changes in ipv4_pktinfo_prepare function. use it directly
   and copy the control block.

Fixes: d826eb1 ("ipv4: PKTINFO doesnt need dst reference")
Signed-off-by: Leone Fernando <leone4fernando@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ipmr.c

@@ -1025,6 +1025,10 @@ static int ipmr_cache_report(const struct mr_table *mrt,
 	struct sk_buff *skb;
 	int ret;
 
+	mroute_sk = rcu_dereference(mrt->mroute_sk);
+	if (!mroute_sk)
+		return -EINVAL;
+
 	if (assert == IGMPMSG_WHOLEPKT || assert == IGMPMSG_WRVIFWHOLE)
 		skb = skb_realloc_headroom(pkt, sizeof(struct iphdr));
 	else
@@ -1069,7 +1073,8 @@ static int ipmr_cache_report(const struct mr_table *mrt,
 		msg = (struct igmpmsg *)skb_network_header(skb);
 		msg->im_vif = vifi;
 		msg->im_vif_hi = vifi >> 8;
-		skb_dst_set(skb, dst_clone(skb_dst(pkt)));
+		ipv4_pktinfo_prepare(mroute_sk, pkt);
+		memcpy(skb->cb, pkt->cb, sizeof(skb->cb));
 		/* Add our header */
 		igmp = skb_put(skb, sizeof(struct igmphdr));
 		igmp->type = assert;
@@ -1079,12 +1084,6 @@ static int ipmr_cache_report(const struct mr_table *mrt,
 		skb->transport_header = skb->network_header;
 	}
 
-	mroute_sk = rcu_dereference(mrt->mroute_sk);
-	if (!mroute_sk) {
-		kfree_skb(skb);
-		return -EINVAL;
-	}
-
 	igmpmsg_netlink_event(mrt, skb);
 
 	/* Deliver to mrouted */