[TCP]: Unconditionally clear TCP_NAGLE_PUSH in skb_entail().

Intention of this bit is to force pushing of the existing
send queue when TCP_CORK or TCP_NODELAY state changes via
setsockopt().

But it's easy to create a situation where the bit never
clears.  For example, if the send queue starts empty:

1) set TCP_NODELAY
2) clear TCP_NODELAY
3) set TCP_CORK
4) do small write()

The current code will leave TCP_NAGLE_PUSH set after that
sequence.  Unconditionally clearing the bit when new data
is added via skb_entail() solves the problem.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp.c

@@ -584,7 +584,7 @@ static inline void skb_entail(struct sock *sk, struct tcp_sock *tp,
 	sk_charge_skb(sk, skb);
 	if (!sk->sk_send_head)
 		sk->sk_send_head = skb;
-	else if (tp->nonagle&TCP_NAGLE_PUSH)
+	if (tp->nonagle & TCP_NAGLE_PUSH)
 		tp->nonagle &= ~TCP_NAGLE_PUSH; 
 }