This was added in commit 46d3ceabd8d98ed0ad10f20c595ca784e34786c5 (tcp: 
TCP Small Queues) but we need it for pppoatm too.

--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -858,6 +858,8 @@ struct proto {
 	int			(*backlog_rcv) (struct sock *sk,
 						struct sk_buff *skb);
 
+	void		(*release_cb)(struct sock *sk);
+
 	/* Keeping track of sk's, looking them up, and port selection methods. */
 	void			(*hash)(struct sock *sk);
 	void			(*unhash)(struct sock *sk);
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -2159,6 +2159,10 @@ void release_sock(struct sock *sk)
 	spin_lock_bh(&sk->sk_lock.slock);
 	if (sk->sk_backlog.tail)
 		__release_sock(sk);
+
+	if (sk->sk_prot->release_cb)
+		sk->sk_prot->release_cb(sk);
+
 	sk->sk_lock.owned = 0;
 	if (waitqueue_active(&sk->sk_lock.wq))
 		wake_up(&sk->sk_lock.wq);