wireguard: netlink: validate nested arrays in policy

wireguard: netlink: validate nested arrays in policy

Use NLA_POLICY_NESTED_ARRAY() to perform nested array validation
in the policy validation step.

The nested policy was already enforced through nla_parse_nested(),
however extack wasn't passed previously, so no fancy error messages.

With the nested attributes being validated directly in the policy, the
policy argument can be set to NULL in the calls to nla_parse_nested().

Signed-off-by: Asbjørn Sloth Tønnesen <ast@fiberby.net>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>