snapshots need additional space reservation while the scrubber works.

[leftwo]

To make a snapshot, we create a destination region first.
After creating the snapshot, while the scrubber copies data from snapshot to the
eventual backing downstairs region, we need a temporary space reservation in
Nexus to allow for the original volume to be written to without running out of space.

Due to the copy on write snapshot feature of ZFS, it's possible for the original volume
to take writes that will grow the total space required for snapshot+source in a way we
don't currently account for.