xfrm: espintcp: save and call old ->sk_destruct

When ESP encapsulation is enabled on a TCP socket, I'm replacing the
existing ->sk_destruct callback with espintcp_destruct. We still need to
call the old callback to perform the other cleanups when the socket is
destroyed. Save the old callback, and call it from espintcp_destruct.

Fixes: e27cca96cd ("xfrm: add espintcp (RFC 8229)")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

include/net/espintcp.h

@@ -25,6 +25,7 @@ struct espintcp_ctx {
 	struct espintcp_msg partial;
 	void (*saved_data_ready)(struct sock *sk);
 	void (*saved_write_space)(struct sock *sk);
+	void (*saved_destruct)(struct sock *sk);
 	struct work_struct work;
 	bool tx_running;
 };

net/xfrm/espintcp.c

@@ -379,6 +379,7 @@ static void espintcp_destruct(struct sock *sk)
 {
 	struct espintcp_ctx *ctx = espintcp_getctx(sk);
 
+	ctx->saved_destruct(sk);
 	kfree(ctx);
 }
 
@@ -419,6 +420,7 @@ static int espintcp_init_sk(struct sock *sk)
 	sk->sk_socket->ops = &espintcp_ops;
 	ctx->saved_data_ready = sk->sk_data_ready;
 	ctx->saved_write_space = sk->sk_write_space;
+	ctx->saved_destruct = sk->sk_destruct;
 	sk->sk_data_ready = espintcp_data_ready;
 	sk->sk_write_space = espintcp_write_space;
 	sk->sk_destruct = espintcp_destruct;