fix file.diskusage for non-directories & avoid shadowing `stat` module

This is exactly why shadowing is dangerous!

```
************* Module salt.modules.file
salt/modules/file.py:4334: [W0621(redefined-outer-name), diskusage] Redefining name 'stat' from outer scope (line 28)
salt/modules/file.py:4326: [E1101(no-member), diskusage] Module 'stat' has no 'st_size' member
```