linprocfs: Avoid using a sysctl to get the CPU model string

This will fail if the reading process is in capability mode. Just copy
the string directly.

PR: 276043
Reviewed by: emaste, imp, kib
Reported and tested by: Ricardo Branco <rbranco@suse.com>
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D43281