bsdinstall: Consistently use item count as the first argument to calloc

Reported by: GCC 14 -Wcalloc-transposed-args
Reviewed by: rlibby, imp, emaste
Differential Revision: https://reviews.freebsd.org/D46012

(cherry picked from commit 7cd0a4c85dbe5e8cd000f6b293ef2d579d22edfb)