/root could have a quota

[citrus-it]

We've had a couple of occasions where the creation of a tar file within the ramdisk rpool has caused it to fill up and consequently affect services running on the system. https://github.com/oxidecomputer/customer-support/issues/52 has a write up on one of them with links to issues that were created as a result, including #130.

On the latest occasion, it appears that a large file was created in root's home directory and, once space was exhausted, removed again. In the interim the sled-agent, mg-ddm and pumkind services in the global zone went into maintenance.

Applying a modest quota to root's home directory, /root would add some guard rails here.