When an IPv6 packet is received for a raw socket which has the
IPPROTO_IPV6 level socket option IPV6_CHECKSUM enabled and the
checksum check fails, drop the message. Without this fix, an
ICMP6 message was sent indicating a parameter problem.

Thanks to bz@ for suggesting a way to simplify this fix.

Reviewed by: bz@
MFC after: 1 week
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D19969