Improve input validation for the socket option IPV6_CHECKSUM.

When using the IPPROTO_IPV6 level socket option IPV6_CHECKSUM on a raw
IPv6 socket, ensure that the value is either -1 or a non-negative even
number.

Reviewed by: bz@, thj@
MFC after: 1 week
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D19966