mptcp: drop sndr_key in mptcp_syn_options

In RFC 8684, we don't need to send sndr_key in SYN package anymore, so drop
it.

Fixes: cc7972ea ("mptcp: parse and emit MP_CAPABLE option according to v1 spec")
Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mptcp/options.c

@@ -336,9 +336,7 @@ bool mptcp_syn_options(struct sock *sk, const struct sk_buff *skb,
 	 */
 	subflow->snd_isn = TCP_SKB_CB(skb)->end_seq;
 	if (subflow->request_mptcp) {
-		pr_debug("local_key=%llu", subflow->local_key);
 		opts->suboptions = OPTION_MPTCP_MPC_SYN;
-		opts->sndr_key = subflow->local_key;
 		*size = TCPOLEN_MPTCP_MPC_SYN;
 		return true;
 	} else if (subflow->request_join) {