devstat: Provide 32-bit compatibility

If a 32-bit process running on a 64-bit kernel requests kern.devstat.all,
translate each struct devstat to its 32-bit equivalent before copying it
out.

Also fix a bug where an early error would be ignored if there were no
devices to report.

MFC after: 1 week
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D54591