ipv4: make tcp_notsent_lowat sysctl knob behave as true unsigned int

> cat /proc/sys/net/ipv4/tcp_notsent_lowat
-1
> echo 4294967295 > /proc/sys/net/ipv4/tcp_notsent_lowat
-bash: echo: write error: Invalid argument
> echo -2147483648 > /proc/sys/net/ipv4/tcp_notsent_lowat
> cat /proc/sys/net/ipv4/tcp_notsent_lowat
-2147483648

but in documentation we have "tcp_notsent_lowat - UNSIGNED INTEGER"

v2: simplify to just proc_douintvec
Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/sysctl_net_ipv4.c

@@ -951,7 +951,7 @@ static struct ctl_table ipv4_net_table[] = {
 		.data		= &init_net.ipv4.sysctl_tcp_notsent_lowat,
 		.maxlen		= sizeof(unsigned int),
 		.mode		= 0644,
-		.proc_handler	= proc_dointvec,
+		.proc_handler	= proc_douintvec,
 	},
 	{
 		.procname	= "tcp_tw_reuse",