mptcp: use OPTION_MPTCP_MPJ_SYNACK in subflow_finish_connect()

subflow_finish_connect() uses four fields (backup, join_id, thmac, none)
that may contain garbage unless OPTION_MPTCP_MPJ_SYNACK has been set
in mptcp_parse_option()

Fixes: f296234c ("mptcp: Add handling of incoming MP_JOIN requests")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Florian Westphal <fw@strlen.de>
Cc: Peter Krystad <peter.krystad@linux.intel.com>
Cc: Matthieu Baerts <matttbe@kernel.org>
Cc: Mat Martineau <martineau@kernel.org>
Cc: Geliang Tang <geliang.tang@linux.dev>
Reviewed-by: Simon Horman <horms@kernel.org>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20240111194917.4044654-4-edumazet@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/subflow.c

@@ -506,7 +506,7 @@ static void subflow_finish_connect(struct sock *sk, const struct sk_buff *skb)
 	} else if (subflow->request_join) {
 		u8 hmac[SHA256_DIGEST_SIZE];
 
-		if (!(mp_opt.suboptions & OPTIONS_MPTCP_MPJ)) {
+		if (!(mp_opt.suboptions & OPTION_MPTCP_MPJ_SYNACK)) {
 			subflow->reset_reason = MPTCP_RST_EMPTCP;
 			goto do_reset;
 		}