inet6: Support RFC 4862 5.5.3.e in regards to Valid Lifetime

inet6: Support RFC 4862 5.5.3.e in regards to Valid Lifetime

      1.  If the received Valid Lifetime is greater than 2 hours or
          greater than RemainingLifetime, set the valid lifetime of the
          corresponding address to the advertised Valid Lifetime.

      2.  If RemainingLifetime is less than or equal to 2 hours, ignore
          the Prefix Information option with regards to the valid
          lifetime.

      3.  Otherwise, reset the valid lifetime of the corresponding
          address to 2 hours.