tcp: tcp_data_ready() must look at SOCK_DONE

My prior cleanup missed that tcp_data_ready() has to look at SOCK_DONE.
Otherwise, an application using SO_RCVLOWAT will not get EPOLLIN event
if a FIN is received in the middle of expected payload.

The reason SOCK_DONE is not examined in tcp_epollin_ready()
is that tcp_poll() catches the FIN because tcp_fin()
is also setting RCV_SHUTDOWN into sk->sk_shutdown

Fixes: 05dc72a ("tcp: factorize logic into tcp_epollin_ready()")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Wei Wang <weiwan@google.com>
Cc: Arjun Roy <arjunroy@google.com>
Reviewed-by: Wei Wang <weiwan@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -4924,7 +4924,7 @@ int tcp_send_rcvq(struct sock *sk, struct msghdr *msg, size_t size)
 
 void tcp_data_ready(struct sock *sk)
 {
-	if (tcp_epollin_ready(sk, sk->sk_rcvlowat))
+	if (tcp_epollin_ready(sk, sk->sk_rcvlowat) || sock_flag(sk, SOCK_DONE))
 		sk->sk_data_ready(sk);
 }