tcp: increase size at which tcp_bound_to_half_wnd bounds to > TCP_MSS_DEFAULT

In previous commit 01f83d69
the following comments were added:

"When peer uses tiny windows, there is no use in packetizing to sub-MSS
pieces for the sake of SWS or making sure there are enough packets in
the pipe for fast recovery."

The test should be > TCP_MSS_DEFAULT not >= 512. This allows low end
devices that send an MSS of 536 (TCP_MSS_DEFAULT) to see better network
performance by sending it 536 bytes of data at a time instead of bounding
to half window size (268). Other network stacks work this way, e.g. HP-UX.
Signed-off-by: NShane Seymour <shane.seymour@hpe.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/tcp.h

@@ -589,7 +589,7 @@ static inline int tcp_bound_to_half_wnd(struct tcp_sock *tp, int pktsize)
 	 * On the other hand, for extremely large MSS devices, handling
 	 * smaller than MSS windows in this way does make sense.
 	 */
-	if (tp->max_window >= 512)
+	if (tp->max_window > TCP_MSS_DEFAULT)
 		cutoff = (tp->max_window >> 1);
 	else
 		cutoff = tp->max_window;