Apply r280991 to ip6_fragment.

This uses m_dup_pkthdr() to copy all of the metadata about a packet to
each of its fragments including VLAN tags, mbuf tags, etc. instead of
hand-copying a few fields.

Reviewed by: bz
MFC after: 1 month
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D20117