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;
 };