Additional length check for IPv6 reassembled fragments.

Additional length check for IPv6 reassembled fragments.

FreeBSD-SA-23:06.ipv6 security advisory has added an additional
overflow check in frag6_input().  OpenBSD is not affected by that
as the bug was introduced by another change in 2019.  The existing
code is complicated and NetBSD has taken the FreeBSD fix, although
they were also not affected.

The additional check makes the complicated code more robust.  Length
calculation taken from NetBSD.  Discussed with FreeBSD.

OK sashan@ mvs@