tcp: tcp_send_challenge_ack delete useless param `skb`

After this parameter is passed in, there is no usage, and deleting it will
not bring any impact.

Reviewed-by: Eric Dumazet <[e-mail]>
Signed-off-by: Benjamin Yim <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/ipv4/tcp_input.c

···
 }
 
 /* RFC 5961 7 [ACK Throttling] */
-static void tcp_send_challenge_ack(struct sock *sk, const struct sk_buff *skb)
+static void tcp_send_challenge_ack(struct sock *sk)
 {
 	/* unprotected vars, we dont care of overwrites */
 	static u32 challenge_timestamp;
···
 		/* RFC 5961 5.2 [Blind Data Injection Attack].[Mitigation] */
 		if (before(ack, prior_snd_una - tp->max_window)) {
 			if (!(flag & FLAG_NO_CHALLENGE_ACK))
-				tcp_send_challenge_ack(sk, skb);
+				tcp_send_challenge_ack(sk);
 			return -1;
 		}
 		goto old_ack;
···
 			if (tp->syn_fastopen && !tp->data_segs_in &&
 			    sk->sk_state == TCP_ESTABLISHED)
 				tcp_fastopen_active_disable(sk);
-			tcp_send_challenge_ack(sk, skb);
+			tcp_send_challenge_ack(sk);
 		}
 		goto discard;
 	}
···
 		if (syn_inerr)
 			TCP_INC_STATS(sock_net(sk), TCP_MIB_INERRS);
 		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPSYNCHALLENGE);
-		tcp_send_challenge_ack(sk, skb);
+		tcp_send_challenge_ack(sk);
 		goto discard;
 	}
 
···
 	if (!acceptable) {
 		if (sk->sk_state == TCP_SYN_RECV)
 			return 1;	/* send one RST */
-		tcp_send_challenge_ack(sk, skb);
+		tcp_send_challenge_ack(sk);
 		goto discard;
 	}
 	switch (sk->sk_state) {