Fix "ValidationErrors object has no attribute check" traceback

Running FreeNAS-11.2-INTERNAL-7.  While testing https://jira.ixsystems.com/browse/NAS-101755?filter=10744  was unable to access mountpoints of any plugins/jails created.  Both on a VM and a FreeNAS Mini.  Verified on another system (Joe Maloney) that the same trace back was received.

Error: concurrent.futures.process._RemoteTraceback: 
"""
Traceback (most recent call last):
File "/usr/local/lib/python3.6/concurrent/futures/process.py", line 175, in _process_worker
r = call_item.fn(*call_item.args, **call_item.kwargs)
File "/usr/local/lib/python3.6/site-packages/middlewared/worker.py", line 133, in main_worker
res = loop.run_until_complete(coro)
File "/usr/local/lib/python3.6/asyncio/base_events.py", line 484, in run_until_complete
return future.result()
File "/usr/local/lib/python3.6/site-packages/middlewared/worker.py", line 93, in _run
return await self._call(name, serviceobj, methodobj, params=args, job=job)
File "/usr/local/lib/python3.6/site-packages/middlewared/worker.py", line 85, in _call
return methodobj(*params)
File "/usr/local/lib/python3.6/site-packages/middlewared/worker.py", line 85, in _call
return methodobj(*params)
File "/usr/local/lib/python3.6/site-packages/middlewared/schema.py", line 668, in nf
return f(*args, **kwargs)
File "/usr/local/lib/python3.6/site-packages/middlewared/plugins/jail.py", line 683, in fstab
verrors.check()
AttributeError: 'ValidationErrors' object has no attribute 'check'
"""

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
File "/usr/local/lib/python3.6/site-packages/middlewared/main.py", line 167, in call_method
result = await self.middleware.call_method(self, message)
File "/usr/local/lib/python3.6/site-packages/middlewared/main.py", line 1098, in call_method
return await self._call(message['method'], serviceobj, methodobj, params, app=app, io_thread=False)
File "/usr/local/lib/python3.6/site-packages/middlewared/main.py", line 1043, in _call
return await self._call_worker(name, *args)
File "/usr/local/lib/python3.6/site-packages/middlewared/main.py", line 1063, in _call_worker
return await self.run_in_proc(main_worker, name, args, job)
File "/usr/local/lib/python3.6/site-packages/middlewared/main.py", line 997, in run_in_proc
return await self.run_in_executor(self.__procpool, method, *args, **kwargs)
File "/usr/local/lib/python3.6/site-packages/middlewared/main.py", line 973, in run_in_executor
return await loop.run_in_executor(pool, functools.partial(method, *args, **kwargs))
AttributeError: 'ValidationErrors' object has no attribute 'check'