ot::Ip6::Nd::RouteInfoOption::OptionLengthForPrefix() method

Calculates the minimum option length for a given prefix length. The option length (which is in unit of 8 octets) can be 1, 2, or 3 depending on the prefix length. It can be 1 for a zero prefix length, 2 if the prefix length is not greater than 64, and 3 otherwise.