rtsold: Fix validation of RDNSS options

The header specifies the size of the option in multiples of eight bytes.
The option consists of an eight-byte header followed by one or more IPv6
addresses, so the option is invalid if the size is not equal to 1+2n for
some n>0. Check this.

The bug can cause random stack data to be formatted as an IPv6 address
and passed to resolvconf(8), but a host able to trigger the bug may also
specify arbitrary addresses this way.

Reported by: Q C <cq674350529@gmail.com>
Sponsored by: The FreeBSD Foundation
MFC after: 3 days