mptcp: avoid additional indirection in mptcp_poll()

We are going to remove the first subflow socket soon, so avoid
the additional indirection at poll() time. Instead access
directly the first subflow sock.

No functional changes intended.
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

@@ -3844,12 +3844,12 @@ static __poll_t mptcp_poll(struct file *file, struct socket *sock,
 	state = inet_sk_state_load(sk);
 	pr_debug("msk=%p state=%d flags=%lx", msk, state, msk->flags);
 	if (state == TCP_LISTEN) {
-		struct socket *ssock = READ_ONCE(msk->subflow);
+		struct sock *ssk = READ_ONCE(msk->first);
 
-		if (WARN_ON_ONCE(!ssock || !ssock->sk))
+		if (WARN_ON_ONCE(!ssk))
 			return 0;
 
-		return inet_csk_listen_poll(ssock->sk);
+		return inet_csk_listen_poll(ssk);
 	}
 
 	shutdown = READ_ONCE(sk->sk_shutdown);