Simplify overlay.{c,h}

Simplify overlay.{c,h}

The struct

	struct ovl_rsync_data {
		struct bdev *orig;
		struct bdev *new;
	};

is simply a duplicate of

	struct rsync_data {
		struct bdev *orig;
		struct bdev *new;
	};

So let's replace any references to ovl_rsync_data with rsync_data.

Signed-off-by: Christian Brauner <christian.brauner@mailbox.org>