[NETLINK]: Fix ACK processing after netlink_dump_start

Revert to original netlink behavior. Do not reply with ACK if the
netlink dump has bees successfully started.

libnl has been broken by the cd40b7d3983c708aabe3d3008ec64ffce56d33b0
The following command reproduce the problem:
   /nl-route-get 192.168.1.1

Signed-off-by: Denis V. Lunev <den@openvz.org>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index 98e313e..3252729 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -1565,7 +1565,11 @@
 
 	netlink_dump(sk);
 	sock_put(sk);
-	return 0;
+
+	/* We successfully started a dump, by returning -EINTR we
+	 * signal not to send ACK even if it was requested.
+	 */
+	return -EINTR;
 }
 
 void netlink_ack(struct sk_buff *in_skb, struct nlmsghdr *nlh, int err)
@@ -1619,17 +1623,21 @@
 
 		/* Only requests are handled by the kernel */
 		if (!(nlh->nlmsg_flags & NLM_F_REQUEST))
-			goto skip;
+			goto ack;
 
 		/* Skip control messages */
 		if (nlh->nlmsg_type < NLMSG_MIN_TYPE)
-			goto skip;
+			goto ack;
 
 		err = cb(skb, nlh);
-skip:
+		if (err == -EINTR)
+			goto skip;
+
+ack:
 		if (nlh->nlmsg_flags & NLM_F_ACK || err)
 			netlink_ack(skb, nlh, err);
 
+skip:
 	        msglen = NLMSG_ALIGN(nlh->nlmsg_len);
 		if (msglen > skb->len)
 			msglen = skb->len;