ipv6: Fix getsockopt() for some IPPROTO_IPV6 level socket options

Fix getsockopt() for the IPPROTO_IPV6 level socket options with the
following names: IPV6_HOPOPTS, IPV6_RTHDR, IPV6_RTHDRDSTOPTS,
IPV6_DSTOPTS, and IPV6_NEXTHOP.

Reviewed by: markj
MFC after: 3 days
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D31458