Plug routing sysctl leaks.

Various structures exported by sysctl_rtsock() contain padding fields
which were not being zeroed.

Reported by: Thomas Barabosch, Fraunhofer FKIE
Reviewed by: ae
MFC after: 3 days
Security: kernel memory disclosure
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D18333