ctl: avoid heap info leak in ctl_request_sense

Previously 3 bytes of data from the heap could be leaked to ctl
consumers.

Reported by: Synacktiv
Reviewed by: asomers, mav
Sponsored by: The Alpha-Omega Project
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46091

(cherry picked from commit db87c98168b1605f067d283fa36a710369c3849d)