Casting list array to fixed size list raises error
When trying to cast a list array to fixed size list, an AttributeError is raised:
> AttributeError: 'pyarrow.lib.FixedSizeListType' object has no attribute 'length'

Steps to reproduce the bug:
```python
import pyarrow as pa
from datasets.table import array_cast

arr = pa.array([[0, 1]])
array_cast(arr, pa.list_(pa.int64(), 2))
```

Stack trace:
```
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-12-6cb90a1d8216> in <module>
      3 
      4 arr = pa.array([[0, 1]])
----> 5 array_cast(arr, pa.list_(pa.int64(), 2))

~/huggingface/datasets/src/datasets/table.py in wrapper(array, *args, **kwargs)
   1802             return pa.chunked_array([func(chunk, *args, **kwargs) for chunk in array.chunks])
   1803         else:
-> 1804             return func(array, *args, **kwargs)
   1805 
   1806     return wrapper

~/huggingface/datasets/src/datasets/table.py in array_cast(array, pa_type, allow_primitive_to_str, allow_decimal_to_str)
   1920                 else:
   1921                     array_values = array.values[
-> 1922                         array.offset * pa_type.length : (array.offset + len(array)) * pa_type.length
   1923                     ]
   1924                     return pa.FixedSizeListArray.from_arrays(_c(array_values, pa_type.value_type), pa_type.list_size)

AttributeError: 'pyarrow.lib.FixedSizeListType' object has no attribute 'length'
```