altq: Clear stats structures in get_class_stats()

These structures are copied out to userspace, and it's possible to leak
uninitialized stack bytes since these routines and their callers weren't
careful to clear them first. Add memsets to avoid this.

Reported by: Ilja Van Sprundel <ivansprundel@ioactive.com>
Reviewed by: kp, emaste
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D53342

(cherry picked from commit ff08916e9ac689e6ce734de72325fc2bd9495a35)