Fix several dev_clone callbacks to avoid out-of-bounds reads

Use strncmp() instead of bcmp(), so that we don't have to find the
minimum of the string lengths before comparing.

Reviewed by: kib
Reported by: KASAN
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D29463

(cherry picked from commit 3428b6c050d102ba7f95514b29f4f5685d76b645)