BUG: Passing `level=` to `DataFrame.reindex` fails when the index is flat
- [ X] I have checked that this issue has not already been reported.

- [ X] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

---

#### Code Sample, a copy-pastable example

```python
import numpy as np
import pandas as pd

df = pd.DataFrame(np.random.randn(100,40), index=pd.Index(np.arange(100), name='foo'))
new_index = pd.Index(np.arange(50), name='foo')
df.reindex(new_index, level='foo')

```

#### Problem description
Currently the above raises `AttributeError: 'Int64Index' object has no attribute 'levels'`.

The behavior of `DataFrame.reindex` should be consistent between flat Indexes and MultiIndexes - as long the level is valid, I shouldn't have to check whether the index is a flat index or a MultiIndex before deciding whether to pass `level=`. 

#### Expected Output
Same as `df.reindex(new_index)`

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.6.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.3.0-53-generic
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : C.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.0.5
numpy            : 1.18.4
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.1.1
setuptools       : 47.3.1.post20200622
Cython           : 0.29.19
pytest           : None
hypothesis       : None
sphinx           : 3.1.1
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.4.1
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.15.0
pandas_datareader: None
bs4              : 4.9.0
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.4.1
matplotlib       : 3.1.3
numexpr          : 2.7.1
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : None
pyxlsb           : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : None
tables           : 3.4.4
tabulate         : None
xarray           : None
xlrd             : 1.2.0
xlwt             : None
xlsxwriter       : None
numba            : 0.48.0

</details>
