libgeom: 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/D46006
(cherry picked from commit bf3b889a5be141f3abadcfc5a1d1cbdc50273d72)