Fix kerberos error

PR: https://github.com/truenas/middleware/pull/6863

https://jira.ixsystems.com/browse/NAS-110214
```
Traceback (most recent call last):
File "/usr/local/lib/python3.8/site-packages/middlewared/job.py", line 367, in run
await self.future
File "/usr/local/lib/python3.8/site-packages/middlewared/job.py", line 403, in __run_body
rv = await self.method(*([self] + args))
File "/usr/local/lib/python3.8/site-packages/middlewared/plugins/smb.py", line 473, in configure
await self.middleware.call("directoryservices.initialize")
File "/usr/local/lib/python3.8/site-packages/middlewared/main.py", line 1238, in call
return await self._call(
File "/usr/local/lib/python3.8/site-packages/middlewared/main.py", line 1195, in _call
return await methodobj(*prepared_call.args)
File "/usr/local/lib/python3.8/site-packages/middlewared/plugins/directoryservices.py", line 416, in initialize
await self.middleware.call('kerberos.start')
File "/usr/local/lib/python3.8/site-packages/middlewared/main.py", line 1238, in call
return await self._call(
File "/usr/local/lib/python3.8/site-packages/middlewared/main.py", line 1195, in _call
return await methodobj(*prepared_call.args)
File "/usr/local/lib/python3.8/site-packages/middlewared/plugins/kerberos.py", line 577, in start
await asyncio.wait_for(self._kinit(), timeout=kinit_timeout)
File "/usr/local/lib/python3.8/asyncio/tasks.py", line 494, in wait_for
return fut.result()
File "/usr/local/lib/python3.8/site-packages/middlewared/plugins/kerberos.py", line 322, in _kinit
await self.do_kinit(ldap)
File "/usr/local/lib/python3.8/site-packages/middlewared/plugins/kerberos.py", line 273, in do_kinit
elif dstype == DSType.LDAP:
File "/usr/local/lib/python3.8/enum.py", line 384, in _getattr_
raise AttributeError(name) from None
AttributeError: LDAP
```