Dataset accounting: Probe zone filesystems should be explicitly requested

[smklein]

Currently, the sled agent picks a random U.2 to hold the zone filesystem for probes

omicron/sled-agent/src/probe_manager.rs

Lines 282 to 291 in 89b998c

	let mut rng = rand::rngs::StdRng::from_entropy();
	let current_disks = self
	.storage
	.get_latest_disks()
	.await
	.all_u2_mountpoints(ZONE_DATASET);
	let zone_root_path = current_disks
	.into_iter()
	.choose(&mut rng)
	.ok_or_else(|| anyhow!("u2 not found"))?;

This, unfortunately, leaves nexus out-of-the-loop regarding space allocation within the zone dataset.

Nexus should be aware of where instances are provisioned to manage accounting explicitly.

[smklein]

See also: #6108