Avoid kernel stack disclosure in compat32 stat

copy_stat and copy_ostat used by 32-bit compat *stat calls left spare
and padding bytes uninitialized.

This issue does not exist in stable/12 and later as it was incidentally
addressed as part of the 64-bit inode project.

Reported by: Reno Robert of Trend Micro Zero Day Initiative
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34706