net: tls, fix sk_write_space NULL write when tx disabled

The ctx->sk_write_space pointer is only set when TLS tx mode is enabled.
When running without TX mode its a null pointer but we still set the
sk sk_write_space pointer on close().

Fix the close path to only overwrite sk->sk_write_space when the current
pointer is to the tls_write_space function indicating the tls module should
clean it up properly as well.
Reported-by: Hillf Danton <hdanton@sina.com>
Cc: Ying Xue <ying.xue@windriver.com>
Cc: Andrey Konovalov <andreyknvl@google.com>
Fixes: 57c722e9 ("net/tls: swap sk_write_space on close")
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/tls/tls_main.c

@@ -308,6 +308,7 @@ static void tls_sk_proto_close(struct sock *sk, long timeout)
 	if (free_ctx)
 		icsk->icsk_ulp_data = NULL;
 	sk->sk_prot = ctx->sk_proto;
+	if (sk->sk_write_space == tls_write_space)
 		sk->sk_write_space = ctx->sk_write_space;
 	write_unlock_bh(&sk->sk_callback_lock);
 	release_sock(sk);