fs/squashfs: Set ret to 0 on successful read

It might still be a positive number due to the call to sqfs_disk_read.
This only happens when reading a file from an uncompressed squashfs.

I found this by trying to boot using the extlinux bootmethod, where
positive values are treated as errors.

Signed-off-by: Michael Zimmermann <sigmaepsilon92@gmail.com>
Acked-by: Richard Genoud <richard.genoud@bootlin.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

fs/squashfs/sqfs.c

@@ -1636,6 +1636,8 @@ static int sqfs_read_nest(const char *filename, void *buf, loff_t offset,
 		*actread = finfo.size;
 	}
 
+	ret = 0;
+
 out:
 	free(fragment);
 	free(datablock);