mptcp: avoid subflow socket usage in mptcp_get_port()

We are going to remove the first subflow socket soon, so avoid
accessing it in mptcp_get_port(). Instead, access directly the
first subflow sock.
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mptcp/protocol.c

@@ -3405,14 +3405,12 @@ static void mptcp_unhash(struct sock *sk)
 static int mptcp_get_port(struct sock *sk, unsigned short snum)
 {
 	struct mptcp_sock *msk = mptcp_sk(sk);
-	struct socket *ssock;
 
-	ssock = msk->subflow;
-	pr_debug("msk=%p, subflow=%p", msk, ssock);
-	if (WARN_ON_ONCE(!ssock))
+	pr_debug("msk=%p, ssk=%p", msk, msk->first);
+	if (WARN_ON_ONCE(!msk->first))
 		return -EINVAL;
 
-	return inet_csk_get_port(ssock->sk, snum);
+	return inet_csk_get_port(msk->first, snum);
 }
 
 void mptcp_finish_connect(struct sock *ssk)