MFC r328876:
Modify ip6_get_prevhdr() to be able use it safely.

Instead of returning pointer to the previous header, return its offset.
In frag6_input() use m_copyback() and determined offset to store next
header instead of accessing to it by pointer and assuming that the memory
is contiguous.

In rip6_input() use offset returned by ip6_get_prevhdr() instead of
calculating it from pointers arithmetic, because IP header can belong
to another mbuf in the chain.

Reported by: Maxime Villard <max at m00nbsd dot net>

MFC r314430:
When IPv6 fragments reassembly is complete, update mbuf's csum_data
and csum_flags using information from all fragments. This fixes
dropping of reassembled packets due to wrong checksum when the IPv6
checksum offloading is enabled on a network card.

Merge r278828, r278832

- Factor out ip6_deletefraghdr() function, to be shared between IPv6 stack and pf(4).
- Move ip6_deletefraghdr() to frag6.c. (Suggested by bz)

Differential Revision: https://reviews.freebsd.org/D2813
Reviewed by: gnn

Merge r278828, r278832

- Factor out ip6_deletefraghdr() function, to be shared between IPv6 stack and pf(4).
- Move ip6_deletefraghdr() to frag6.c. (Suggested by bz)

Differential Revision: https://reviews.freebsd.org/D2813
Reviewed by: gnn