net-tcp: /proc/sys/net/ipv4/tcp_probe_interval is a u32 not int

(fix documentation and sysctl access to treat it as such)

Tested:
  # zcat /proc/config.gz | egrep ^CONFIG_HZ
  CONFIG_HZ_1000=y
  CONFIG_HZ=1000
  # echo $[(1<<32)/1000 + 1] | tee /proc/sys/net/ipv4/tcp_probe_interval
  4294968
  tee: /proc/sys/net/ipv4/tcp_probe_interval: Invalid argument
  # echo $[(1<<32)/1000] | tee /proc/sys/net/ipv4/tcp_probe_interval
  4294967
  # echo 0 | tee /proc/sys/net/ipv4/tcp_probe_interval
  # echo -1 | tee /proc/sys/net/ipv4/tcp_probe_interval
  -1
  tee: /proc/sys/net/ipv4/tcp_probe_interval: Invalid argument

Signed-off-by: Maciej Żenczykowski <maze@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -425,7 +425,7 @@ tcp_mtu_probing - INTEGER
 	  1 - Disabled by default, enabled when an ICMP black hole detected
 	  2 - Always enabled, use initial MSS of tcp_base_mss.
 
-tcp_probe_interval - INTEGER
+tcp_probe_interval - UNSIGNED INTEGER
 	Controls how often to start TCP Packetization-Layer Path MTU
 	Discovery reprobe. The default is reprobing every 10 minutes as
 	per RFC4821.

net/ipv4/sysctl_net_ipv4.c

@@ -48,6 +48,7 @@ static int tcp_syn_retries_max = MAX_TCP_SYNCNT;
 static int ip_ping_group_range_min[] = { 0, 0 };
 static int ip_ping_group_range_max[] = { GID_T_MAX, GID_T_MAX };
 static int comp_sack_nr_max = 255;
+static u32 u32_max_div_HZ = UINT_MAX / HZ;
 
 /* obsolete */
 static int sysctl_tcp_low_latency __read_mostly;
@@ -745,9 +746,10 @@ static struct ctl_table ipv4_net_table[] = {
 	{
 		.procname	= "tcp_probe_interval",
 		.data		= &init_net.ipv4.sysctl_tcp_probe_interval,
-		.maxlen		= sizeof(int),
+		.maxlen		= sizeof(u32),
 		.mode		= 0644,
-		.proc_handler	= proc_dointvec,
+		.proc_handler	= proc_douintvec_minmax,
+		.extra2		= &u32_max_div_HZ,
 	},
 	{
 		.procname	= "igmp_link_local_mcast_reports",