Hardcode the loopback rx/tx checkum options for IPv6 to on without
checking. This allows the FreeBSD 9.1 release process to move forward.
Work around the problem that loopback connections to local addresses
not on loopback interfaces and not on interfaces w/ IPv6 checksum offloading
enabled would not work.
A proper fix to allow us to disable the "checksum offload" on loopback
for testing, measurements, ... as we allow for IPv4 needs to put in
place later.

Reported by: tuexen, Matthew Seaman (m.seaman infracaninophile.co.uk)
Reported by: Mike Andrews (mandrews bit0.com), kib, ...
PR: kern/170070
MFC after: 1 day
X-MFC after: re approval