ceph: Use kcalloc for allocating multiple elements

Prefer using kcalloc(a, b) over kzalloc(a * b) as this improves
semantics since kcalloc is intended for allocating an array of memory.

Signed-off-by: Kenneth Lee <klee33@uw.edu>
Reviewed-by: Xiubo Li <xiubli@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>