Correct a kernel stack leak in 32-bit compat when vfc_name is short.

Don't zero unused pointer members again.

Per discussion with secteam we are not issuing an advisory for this
issue as we have no current evidence it leaks exploitable information.

Reviewed by: rwatson, glebius, delphij
MFC after: 1 day
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D10227