sctp: unbalanced rcu lock in ip_queue_xmit()

sctp: unbalanced rcu lock in ip_queue_xmit()

The bug was introduced in 2.6.32.61 by commit b8710128e201 ("inet: add RCU
protection to inet->opt") (it's a backport of upstream commit f6d8bd051c39).

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>