makefs/zfs: Set the objset block pointer fill field correctly

It is supposed to contain the number of objects allocated from the set,
excluding the meta dnode.

Approved by: re (gjb)
Reported by: mav
MFC after: 1 week
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 46402fd21631b7d4686f8c341d92db6bcb190e5d)
(cherry picked from commit ea5173e5725564da4b95a5b98410b8cd43520dab)