libcamera: file: Check files exist()

Ensure that when we check for existence with File() it will only return
if the path leads to a file, and not a directory.

Device nodes which could still be opened by this class are still
supported.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

src/libcamera/file.cpp

@@ -458,7 +458,8 @@ bool File::exists(const std::string &name)
 	if (ret < 0)
 		return false;
 
-	return true;
+	/* Directories can not be handled here, even if they exist. */
+	return !S_ISDIR(st.st_mode);
 }
 
 } /* namespace libcamera */