[netinet6]: Create a new IPv6 netisr which expects the frames to have been verified.

This is required for fragments and encapsulated data (eg tunneling) to be redistributed
to the RSS bucket based on the eventual IPv6 header and protocol (TCP, UDP, etc) header.

• Add an mbuf tag with the state of IPv6 options parsing before the frame is queued into the direct dispatch handler;
• Continue processing and complete the frame reception in the correct RSS bucket / netisr context.

Testing results are in the phabricator review.

Differential Revision: https://reviews.freebsd.org/D3563
Submitted by: Tiwei Bie <btw@mail.ustc.edu.cn>