sctp: unbalanced rcu lock in ip_queue_xmit()

The bug was introduced in 2.6.32.61 by commit b8710128 ("inet: add RCU
protection to inet->opt") (it's a backport of upstream commit f6d8bd05).

In SCTP case, packet is already routed, hence we jump to the label
'packet_routed', but without rcu_read_lock(). After this label,
rcu_read_unlock() is called unconditionally.
Spotted-by: Guo Fengtian <fengtian.guo@6wind.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Willy Tarreau <w@1wt.eu>

net/ipv4/ip_output.c

@@ -320,13 +320,13 @@ int ip_queue_xmit(struct sk_buff *skb, int ipfragok)
 	/* Skip all of this if the packet is already routed,
 	 * f.e. by something like SCTP.
 	 */
+	rcu_read_lock();
 	rt = skb_rtable(skb);
 	if (rt != NULL)
 		goto packet_routed;
 
 	/* Make sure we can route this packet. */
 	rt = (struct rtable *)__sk_dst_check(sk, 0);
-	rcu_read_lock();
 	inet_opt = rcu_dereference(inet->inet_opt);
 	if (rt == NULL) {
 		__be32 daddr;