ipv4:correct description for tcp_max_syn_backlog

Since commit c5ed63d66f24(tcp: fix three tcp sysctls tuning),
sysctl_max_syn_backlog is determined by tcp_hashinfo->ehash_mask,
and the minimal value is 128, and it will increase in proportion to the
memory of machine.
The original description for tcp_max_syn_backlog and sysctl_max_syn_backlog
are out of date.

Changelog:
V2: update description for sysctl_max_syn_backlog

Signed-off-by: Weiping Pan <panweiping3@gmail.com>
Reviewed-by: Shan Wei <shanwei88@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -282,11 +282,11 @@ tcp_max_ssthresh - INTEGER
 	Default: 0 (off)
 
 tcp_max_syn_backlog - INTEGER
-	Maximal number of remembered connection requests, which are
-	still did not receive an acknowledgment from connecting client.
-	Default value is 1024 for systems with more than 128Mb of memory,
-	and 128 for low memory machines. If server suffers of overload,
-	try to increase this number.
+	Maximal number of remembered connection requests, which have not
+	received an acknowledgment from connecting client.
+	The minimal value is 128 for low memory machines, and it will
+	increase in proportion to the memory of machine.
+	If server suffers from overload, try increasing this number.
 
 tcp_max_tw_buckets - INTEGER
 	Maximal number of timewait sockets held by system simultaneously.

net/core/request_sock.c

@@ -26,10 +26,11 @@
  * but then some measure against one socket starving all other sockets
  * would be needed.
  *
- * It was 128 by default. Experiments with real servers show, that
+ * The minimum value of it is 128. Experiments with real servers show that
  * it is absolutely not enough even at 100conn/sec. 256 cures most
- * of problems. This value is adjusted to 128 for very small machines
- * (<=32Mb of memory) and to 1024 on normal or better ones (>=256Mb).
+ * of problems.
+ * This value is adjusted to 128 for low memory machines,
+ * and it will increase in proportion to the memory of machine.
  * Note : Dont forget somaxconn that may limit backlog too.
  */
 int sysctl_max_syn_backlog = 256;