Provide a read-only property to retrieve number of objects in a dataset.

[iBug]

Describe the feature would like to see added to OpenZFS

A read-only property, similar to used, userused@... and userobjused@..., possibly named objused, to retrieve the number of objects in a dataset (of type filesystem, snapshot or bookmark - it won't make sense for zvols).

How will this feature improve OpenZFS?

Use case: To provide a rough estimate of total number of files (inodes) in a ZFS filesystem, without having to enumerate possible UIDs and reading many userobjused@ properties.

Additional context