Correct some bugs on IPv6 length calculation.

* Packet#recalc now call ipv6_recalc with arg equals to :all or :ipv6.
* IPv6Header#ipv6_recalc now calls itself with arg=:ipv6_len (instead of
  :len) when called with arg=:all.
* IPv6Header#ipv6_calc_len now sets self.ipv6_len instead of self[:ipv6_len].