ctl: fix uninitialized data used by PREVENT ALLOW MEDIUM REMOVAL

Zero-initialize the bitmap of preventers. Otherwise, the START STOP
UNIT command may not have the intended effect.

Sponsored by: Axcient
Reviewed by: emaste, markj
Pull Request: https://github.com/freebsd/freebsd-src/pull/1409

(cherry picked from commit 0f2b2276abc305905e7d88619a7abca26b0dd7eb)