Changed floppy_image_device::init_fs() to set the dirty bit on the floppy image (#9507)

This is not an actual user facing bug right now, because in the MAME UI the call to init_fs() is followed up by a call to setup_write(), which forces the image to commit without regard to the dirty bit.  There is an argument that this is itself code smell; setup_write() perhaps should be set_output_format() and not arbitrarily perform a commit.

src/devices/imagedev/floppy.cpp

@@ -844,6 +844,9 @@ void floppy_image_device::init_fs(const fs_info *fs, const fs::meta_data &meta)
 	} else {
 		fs::unformatted_image::format(fs->m_key, image.get());
 	}
+
+	// intializing a file system makes the floppy dirty
+	image_dirty = true;
 }
 
 /* write protect, active high