[RTNETLINK]: Add rtnetlink notification interface

Adds rtnl_notify() to send rtnetlink notification messages and
rtnl_set_sk_err() to report notification errors as socket
errors in order to indicate the need of a resync due to loss
of events.

nlmsg_report() is added to properly document the meaning of
NLM_F_ECHO.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/rtnetlink.h b/include/linux/rtnetlink.h
index 0e4f478..ecbe034 100644
--- a/include/linux/rtnetlink.h
+++ b/include/linux/rtnetlink.h
@@ -585,6 +585,9 @@
 extern struct rtnetlink_link * rtnetlink_links[NPROTO];
 extern int rtnetlink_send(struct sk_buff *skb, u32 pid, u32 group, int echo);
 extern int rtnl_unicast(struct sk_buff *skb, u32 pid);
+extern int rtnl_notify(struct sk_buff *skb, u32 pid, u32 group,
+		       struct nlmsghdr *nlh, gfp_t flags);
+extern void rtnl_set_sk_err(u32 group, int error);
 extern int rtnetlink_put_metrics(struct sk_buff *skb, u32 *metrics);
 
 extern void __rta_fill(struct sk_buff *skb, int attrtype, int attrlen, const void *data);