[NETLINK]: Do precise netlink message allocations where possible

Account for the netlink message header size directly in nlmsg_new()
instead of relying on the caller calculate it correctly.

Replaces error handling of message construction functions when
constructing notifications with bug traps since a failure implies
a bug in calculating the size of the skb.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Acked-by: Paul Moore <paul.moore@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/decnet/dn_rules.c

@@ -241,6 +241,12 @@ static u32 dn_fib_rule_default_pref(void)
 	return 0;
 }
 
+static size_t dn_fib_rule_nlmsg_payload(struct fib_rule *rule)
+{
+	return nla_total_size(2) /* dst */
+	       + nla_total_size(2); /* src */
+}
+
 int dn_fib_dump_rules(struct sk_buff *skb, struct netlink_callback *cb)
 {
 	return fib_rules_dump(skb, cb, AF_DECnet);