Agent failed to delete a region

Crucible agent had trouble deleting a newly created region:

00:49:47.053Z INFO crucible-agent (dropshot): request completed
    latency_us = 68
    local_addr = [fd00:1122:3344:106::4]:32345
    method = GET
    remote_addr = [fd00:1122:3344:10f::3]:37967
    req_id = 386ab886-b906-4b58-ad9b-09744b267f3a
    response_code = 200
    uri = /crucible/0/regions/623874c5-efc6-4b87-aa1d-cc496e46a12e
00:49:47.395Z INFO crucible-agent (worker): region files created ok
    region = 83ba6068-1df4-49ab-972b-67d36a1aa029
00:49:47.395Z INFO crucible-agent (datafile): region 83ba6068-1df4-49ab-972b-67d36a1aa029 state: Requested -> Created
00:49:47.395Z INFO crucible-agent (worker): applying SMF actions post create...
00:49:47.456Z INFO crucible-agent (worker): disabling downstairs instance: downstairs-623874c5-efc6-4b87-aa1d-cc496e46a12e (instance states: (Some(Online), None))
00:49:47.481Z INFO crucible-agent (worker): creating missing downstairs instance downstairs-83ba6068-1df4-49ab-972b-67d36a1aa029
00:49:47.485Z INFO crucible-agent (worker): ok, have svc:/oxide/crucible/downstairs:downstairs-83ba6068-1df4-49ab-972b-67d36a1aa029
00:49:47.492Z INFO crucible-agent (worker): creating config property group
00:49:47.496Z INFO crucible-agent (worker): reconfiguring svc:/oxide/crucible/downstairs:downstairs-83ba6068-1df4-49ab-972b-67d36a1aa029
00:49:47.497Z INFO crucible-agent (worker): ensure directory SCF_TYPE_ASTRING /data/regions/83ba6068-1df4-49ab-972b-67d36a1aa029
00:49:47.497Z INFO crucible-agent (worker): ensure port SCF_TYPE_COUNT 19002
00:49:47.497Z INFO crucible-agent (worker): ensure address SCF_TYPE_ASTRING fd00:1122:3344:106::4
00:49:47.497Z INFO crucible-agent (worker): commit
00:49:47.499Z INFO crucible-agent (worker): ok!
00:49:47.507Z INFO crucible-agent (worker): SMF ok!
00:49:47.507Z INFO crucible-agent (worker): applying SMF actions before removal...
00:49:47.534Z INFO crucible-agent (worker): disabling downstairs instance: downstairs-623874c5-efc6-4b87-aa1d-cc496e46a12e (instance states: (Some(Online), Some(Disabled)))
00:49:47.538Z INFO crucible-agent (worker): SMF ok!
00:49:47.566Z INFO crucible-agent (worker): deleting zfs dataset "oxp_9b1be26b-6af2-4ec4-8074-9aafadccf73d/crucible/regions/623874c5-efc6-4b87-aa1d-cc496e46a12e"
    region = 623874c5-efc6-4b87-aa1d-cc496e46a12e
00:49:47.610Z INFO crucible-agent (dropshot): request completed
    latency_us = 155
    local_addr = [fd00:1122:3344:106::4]:32345
    method = GET
    remote_addr = [fd00:1122:3344:10e::3]:50908
    req_id = f372ce6d-7510-49e8-90cb-d14814f2d5fb
    response_code = 200
    uri = /crucible/0/regions/623874c5-efc6-4b87-aa1d-cc496e46a12e
00:49:47.623Z ERRO crucible-agent (worker): zfs dataset oxp_9b1be26b-6af2-4ec4-8074-9aafadccf73d/crucible/regions/623874c5-efc6-4b87-aa1d-cc496e46a12e delete attempt 0 failed: out: err:cannot unmount '/data/regions/623874c5-efc6-4b87-aa1d-cc496e46a12e': Device busy
    region = 623874c5-efc6-4b87-aa1d-cc496e46a12e
00:49:50.965Z INFO crucible-agent (dropshot): request completed
    latency_us = 23476
    local_addr = [fd00:1122:3344:106::4]:32345
    method = GET
    remote_addr = [fd00:1122:3344:10f::3]:37967
    req_id = f215d40e-d411-495b-804a-7d4a6e6e1d30
    response_code = 200
    uri = /crucible/0/regions/623874c5-efc6-4b87-aa1d-cc496e46a12e

No source file or test is named. Start with the agent's region-removal path corresponding to the deleting zfs dataset log and trace how the downstairs instance is stopped before deletion. Done means a newly created region can be deleted without the reported Device busy failure.