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: f296234 ("mptcp: Add handling of incoming MP_JOIN requests")
Signed-off-by: Eric Dumazet <[email protected]>
Cc: Florian Westphal <[email protected]>
Cc: Peter Krystad <[email protected]>
Cc: Matthieu Baerts <[email protected]>
Cc: Mat Martineau <[email protected]>
Cc: Geliang Tang <[email protected]>
Reviewed-by: Simon Horman <[email protected]>
Acked-by: Paolo Abeni <[email protected]>
Reviewed-by: Mat Martineau <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: edumazet

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;
 		}