mptcp: generate subflow hmac after mptcp_finish_join()

For outgoing subflow join, when recv SYNACK, in subflow_finish_connect(),
the mptcp_finish_join() may return false in some cases, and send a RESET
to remote, and no local hmac is required.
So generate subflow hmac after mptcp_finish_join().

Fixes: ec3edaa ("mptcp: Add handling of outgoing MP_JOIN requests")
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jianguo Wu <wujianguo@chinatelecom.cn>

net/mptcp/subflow.c

@@ -430,15 +430,15 @@ static void subflow_finish_connect(struct sock *sk, const struct sk_buff *skb)
 			goto do_reset;
 		}
 
+		if (!mptcp_finish_join(sk))
+			goto do_reset;
+
 		subflow_generate_hmac(subflow->local_key, subflow->remote_key,
 				      subflow->local_nonce,
 				      subflow->remote_nonce,
 				      hmac);
 		memcpy(subflow->hmac, hmac, MPTCPOPT_HMAC_LEN);
 
-		if (!mptcp_finish_join(sk))
-			goto do_reset;
-
 		subflow->mp_join = 1;
 		MPTCP_INC_STATS(sock_net(sk), MPTCP_MIB_JOINSYNACKRX);