Fix temporary mount of system dataset during move

When moving the system dataset to a new pool we first temporarily
mount the system dataset in the middleware run directory before
running rsync to copy all of the data to the new path. During
a recent refactor of the system dataset heirarchy the nuance of
the temporary mount point was lost.

src/middlewared/middlewared/plugins/sysdataset.py

@@ -537,10 +537,8 @@ def __mount(self, pool, uuid, path=SYSDATASET_PATH):
         mounted = False
         for ds_config in get_system_dataset_spec(pool, uuid):
             dataset, name = ds_config['name'], os.path.basename(ds_config['name'])
-            if desired_mountpoint := ds_config.get('mountpoint'):
-                mountpoint = desired_mountpoint
-            else:
-                mountpoint = f'{path}/{name}'
+
+            mountpoint = ds_config.get('mountpoint', f'{SYSDATASET_PATH}/{name}').replace(SYSDATASET_PATH, path)
 
             if os.path.ismount(mountpoint):
                 continue

tests/api2/test_006_pool_and_sysds.py

@@ -80,6 +80,16 @@ def test_002_create_permanent_zpool(request, ws_client):
         assert results['pool'] == pool_name
         assert results['basename'] == f'{pool_name}/.system'
 
+    try:
+        sysdataset_update = ws_client.call('core.get_jobs', [
+            ['method', '=', 'systemdataset.update']
+        ], {'order_by': ['-id'], 'get': True})
+    except Exception:
+        fail('Failed to get status of systemdataset update')
+
+    if sysdataset_update['state'] != 'SUCCESS':
+        fail(f'System dataset move failed: {sysdataset_update["error"]}')
+
 
 @pytest.mark.dependency(name='POOL_FUNCTIONALITY1')
 def test_003_verify_unused_disk_and_sysds_functionality_on_2nd_pool(ws_client, pool_data):