Avoid kernel memory disclosure in freebsd32_ioctl_pciocgetconf

This has been addressed in HEAD with other compat32 changes, but was not
yet addressed in stable/11.

admbugs: 797
Reported by: Ilja Van Sprundel <ivansprundel@ioactive.com>
Security: CVE-2019-5605
Sponsored by: The FreeBSD Foundation