tcp: Add tcp_inq to get available receive bytes on socket

Create a common kernel function to get the number of bytes available
on a TCP socket. This is based on code in INQ getsockopt and we now call
the function for that getsockopt.
Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/tcp.h

@@ -1816,4 +1816,28 @@ static inline void skb_set_tcp_pure_ack(struct sk_buff *skb)
 	skb->truesize = 2;
 }
 
+static inline int tcp_inq(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	int answ;
+
+	if ((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV)) {
+		answ = 0;
+	} else if (sock_flag(sk, SOCK_URGINLINE) ||
+		   !tp->urg_data ||
+		   before(tp->urg_seq, tp->copied_seq) ||
+		   !before(tp->urg_seq, tp->rcv_nxt)) {
+
+		answ = tp->rcv_nxt - tp->copied_seq;
+
+		/* Subtract 1, if FIN was received */
+		if (answ && sock_flag(sk, SOCK_DONE))
+			answ--;
+	} else {
+		answ = tp->urg_seq - tp->copied_seq;
+	}
+
+	return answ;
+}
+
 #endif	/* _TCP_H */

net/ipv4/tcp.c

@@ -556,20 +556,7 @@ int tcp_ioctl(struct sock *sk, int cmd, unsigned long arg)
 			return -EINVAL;
 
 		slow = lock_sock_fast(sk);
-		if ((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV))
-			answ = 0;
-		else if (sock_flag(sk, SOCK_URGINLINE) ||
-			 !tp->urg_data ||
-			 before(tp->urg_seq, tp->copied_seq) ||
-			 !before(tp->urg_seq, tp->rcv_nxt)) {
-
-			answ = tp->rcv_nxt - tp->copied_seq;
-
-			/* Subtract 1, if FIN was received */
-			if (answ && sock_flag(sk, SOCK_DONE))
-				answ--;
-		} else
-			answ = tp->urg_seq - tp->copied_seq;
+		answ = tcp_inq(sk);
 		unlock_sock_fast(sk, slow);
 		break;
 	case SIOCATMARK: