[TCP]: Don't over-clamp window in tcp_clamp_window()

From: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>

Handle better the case where the sender sends full sized
frames initially, then moves to a mode where it trickles
out small amounts of data at a time.

This known problem is even mentioned in the comments
above tcp_grow_window() in tcp_input.c, specifically:

...
 * The scheme does not work when sender sends good segments opening
 * window and then starts to feed us spagetti. But it should work
 * in common situations. Otherwise, we have to rely on queue collapsing.
...

When the sender gives full sized frames, the "struct sk_buff" overhead
from each packet is small.  So we'll advertize a larger window.
If the sender moves to a mode where small segments are sent, this
ratio becomes tilted to the other extreme and we start overrunning
the socket buffer space.

tcp_clamp_window() tries to address this, but it's clamping of
tp->window_clamp is a wee bit too aggressive for this particular case.

Fix confirmed by Ion Badulescu.
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -355,8 +355,6 @@ static void tcp_clamp_window(struct sock *sk, struct tcp_sock *tp)
 			app_win -= icsk->icsk_ack.rcv_mss;
 		app_win = max(app_win, 2U*tp->advmss);
 
-		if (!ofo_win)
-			tp->window_clamp = min(tp->window_clamp, app_win);
 		tp->rcv_ssthresh = min(tp->window_clamp, 2U*tp->advmss);
 	}
 }