Commit cd2b7087 ("sctp: check duplicate node before inserting a
new transport") called rhltable_lookup() to check for the duplicate
transport node in transport rhashtable.

But rhltable_lookup() doesn't call rcu_read_lock inside, it could cause
a use-after-free issue if it tries to dereference the node that another
cpu has freed it. Note that sock lock can not avoid this as it is per
sock.

This patch is to fix it by calling rcu_read_lock before checking for
duplicate transport nodes.

Fixes: cd2b7087 ("sctp: check duplicate node before inserting a new transport")
Reported-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>