ocs_fc: Ensure that we zero-initialize memory before copying it out.

Note that the affected interface is available only to root.

admbugs: 765
Reported by: Vlad Tsyrklevich <vlad@tsyrklevich.net>
Reviewed by: emaste, ram
MFC after: 1 day
Security: Kernel memory disclosure
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D18914