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: aaa5f515 ("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>