Preserve IPv6 fragment IDs accross reassembly and refragmentation

When forwarding fragmented IPv6 packets and filtering with PF we
reassemble and refragment. That means we generate new fragment headers
and a new fragment ID.

We already save the fragment IDs so we can do the reassembly so it's
straightforward to apply the incoming fragment ID on the refragmented
packets.

Differential Revision: https://reviews.freebsd.org/D2188
Approved by: gnn (mentor)