net: ipv6: avoid accepting values greater than 2 for accept_untracked_na

The accept_untracked_na sysctl changed from a boolean to an integer
when a new knob '2' was added. This patch provides a safeguard to avoid
accepting values that are not defined in the sysctl. When setting a
value greater than 2, the user will get an 'invalid argument' warning.

Fixes: aaa5f515b1 ("net: ipv6: new accept_untracked_na option to accept na only if in-network")
Signed-off-by: Jaehee Park <jhpark1013@gmail.com>
Suggested-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Suggested-by: Roopa Prabhu <roopa@nvidia.com>
Reviewed-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Link: https://lore.kernel.org/r/20220720183632.376138-1-jhpark1013@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv6/addrconf.c

@@ -7042,9 +7042,9 @@ static const struct ctl_table addrconf_sysctl[] = {
 		.data		= &ipv6_devconf.accept_untracked_na,
 		.maxlen		= sizeof(int),
 		.mode		= 0644,
-		.proc_handler	= proc_dointvec,
-		.extra1		= (void *)SYSCTL_ZERO,
-		.extra2		= (void *)SYSCTL_ONE,
+		.proc_handler	= proc_dointvec_minmax,
+		.extra1		= SYSCTL_ZERO,
+		.extra2		= SYSCTL_TWO,
 	},
 	{
 		/* sentinel */