mksnap_ffs: fix running unded chroot

Fix "run-under-chroot" workaround by replacing strlcpy(2) with
memmove(2) since strings overlap.

Reviewed by: arrowd
Differential Revision: https://reviews.freebsd.org/D52670

(cherry picked from commit 60be0d71daab4247dc61c323777b0c593eb76975)