netlink: ensure to loop over all netns in genlmsg_multicast_allns()

Nowadays, nlmsg_multicast() returns only 0 or -ESRCH but this was not the
case when commit 134e6375 was pushed.
However, there was no reason to stop the loop if a netns does not have
listeners.
Returns -ESRCH only if there was no listeners in all netns.

To avoid having the same problem in the future, I didn't take the
assumption that nlmsg_multicast() returns only 0 or -ESRCH.

Fixes: 134e6375 ("genetlink: make netns aware")
CC: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: NNicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/netlink/genetlink.c

@@ -1081,6 +1081,7 @@ static int genlmsg_mcast(struct sk_buff *skb, u32 portid, unsigned long group,
 {
 	struct sk_buff *tmp;
 	struct net *net, *prev = NULL;
+	bool delivered = false;
 	int err;
 
 	for_each_net_rcu(net) {
@@ -1092,14 +1093,21 @@ static int genlmsg_mcast(struct sk_buff *skb, u32 portid, unsigned long group,
 			}
 			err = nlmsg_multicast(prev->genl_sock, tmp,
 					      portid, group, flags);
-			if (err)
+			if (!err)
+				delivered = true;
+			else if (err != -ESRCH)
 				goto error;
 		}
 
 		prev = net;
 	}
 
-	return nlmsg_multicast(prev->genl_sock, skb, portid, group, flags);
+	err = nlmsg_multicast(prev->genl_sock, skb, portid, group, flags);
+	if (!err)
+		delivered = true;
+	else if (err != -ESRCH)
+		goto error;
+	return delivered ? 0 : -ESRCH;
  error:
 	kfree_skb(skb);
 	return err;