Add readonly zfs property for current mountpoint

[eharris]

Describe the feature would like to see added to OpenZFS

Add a readonly zfs property for where the dataset is currently mounted, since the value may be different (due to zfs set -u mountpoint=) than what the mountpoint property value is. It would also be nice for snapshots.

How will this feature improve OpenZFS?

Ability to easily query where a dataset or snapshot is currently mounted.

[amotin]

I am not sure file system even knows where it is mounted. We had difficulties with finding that on Linux recently, trying to fix .zfs mounts for root dataset after pivot root. We've handled it somehow, but IIRC it was not great.