syscalls: fix modspecific_t stack content leak

Zero-initialize the whole modspecific_t so that there would
not be kernel stack content leak in the unused part.

Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D40815

(cherry picked from commit 2cab2d43b83bddd44ecc1f267afb5e2725975ac7)