devstat: Fix a kernel stack disclosure

The 16-byte "device_name" field was not zero-filled, so could contain
uninitialized stack data. Zero the whole struct, as that's the
prevailing pattern for this kind of conversion code, and it's more
robust in the face of future revisions to struct devstat.

Reviewed by: olce, kib
Reported by: Reo Shiseki
Fixes: a11d132f6c62 ("devstat: Provide 32-bit compatibility")
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D59309

(cherry picked from commit 7cb1a76f88158fb690418336b736e66c238cd4f7)