IPv6: use proto_tree_add_item for IPv6 Routing Header Next and Length

Like other header (Fragment, Hop by Hop...)

Change-Id: I5d0a85bf285b37eed38345d659439400c5f50d06
Reviewed-on: https://code.wireshark.org/review/4634
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>