smb: Allow renaming a file to the same name with a different case

We check to see if the desired file name already exists before doing a renaming by stat'ing the file (to prevent smb from destroying an existing file). Since smb is not case sensitive, the check for an existing file mistakenly returns true if we are only changing the filename's case.

Add a second check to see whether we are simply changing the case of the filename.

Fixes: #672 (closed)