When a checksum has to be computed for a received IPv6 packet because it
is requested by the application using the IPPROTO_IPV6 level socket option
IPV6_CHECKSUM on a raw socket, ensure that the packet contains enough
bytes to contain the checksum at the specified offset.

Reported by: syzbot+6295fcc5a8aced81d599@syzkaller.appspotmail.com
Reviewed by: bz@
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D19968