nfs: Zero the buffers exported by NFSSVC_DUMPCLIENTS and DUMPLOCKS.

Note that these interfaces are available only to root.

admbugs: 765
Reported by: Vlad Tsyrklevich <vlad@tsyrklevich.net>
Reviewed by: rmacklem
MFC after: 1 day
Security: Kernel memory disclosure
Sponsored by: The FreeBSD Foundation