net: increase sk_[max_]ack_backlog

sk_ack_backlog & sk_max_ack_backlog were 16bit fields, meaning
listen() backlog was limited to 65535.

It is time to increase the width to allow much bigger backlog,
if admins change /proc/sys/net/core/somaxconn &
/proc/sys/net/ipv4/tcp_max_syn_backlog default values.

Tested:

echo 5000000 >/proc/sys/net/core/somaxconn
echo 5000000 >/proc/sys/net/ipv4/tcp_max_syn_backlog

Ran a SYNFLOOD test against a listener using listen(fd, 5000000)

myhost~# grep request_sock_TCP /proc/slabinfo
request_sock_TCP  4185642 4411940    304   13    1 : tunables   54   27    8 : slabdata 339380 339380      0
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sock.h

@@ -405,8 +405,8 @@ struct sock {
 	rwlock_t		sk_callback_lock;
 	int			sk_err,
 				sk_err_soft;
-	unsigned short		sk_ack_backlog;
-	unsigned short		sk_max_ack_backlog;
+	u32			sk_ack_backlog;
+	u32			sk_max_ack_backlog;
 	__u32			sk_priority;
 #if IS_ENABLED(CONFIG_CGROUP_NET_PRIO)
 	__u32			sk_cgrp_prioidx;

net/core/sysctl_net_core.c

@@ -24,7 +24,6 @@
 
 static int zero = 0;
 static int one = 1;
-static int ushort_max = USHRT_MAX;
 
 static int net_msg_warn;	/* Unused, but still a sysctl */
 
@@ -401,7 +400,6 @@ static struct ctl_table netns_core_table[] = {
 		.maxlen		= sizeof(int),
 		.mode		= 0644,
 		.extra1		= &zero,
-		.extra2		= &ushort_max,
 		.proc_handler	= proc_dointvec_minmax
 	},
 	{ }