dm cache metadata: use cursor api in blocks_are_clean_separate_dirty()

	bool dirty_flag;

	*result = true;

	// FIXME: use a cursor so we can benefit from preloading metadata.

	r = dm_bitset_cursor_begin(&cmd->dirty_info, cmd->dirty_root,

				   from_cblock(begin), &cmd->dirty_cursor);

	if (r) {

		DMERR("%s: dm_bitset_cursor_begin for dirty failed", __func__);

		return r;

	}

	r = dm_bitset_cursor_skip(&cmd->dirty_cursor, from_cblock(begin));

	if (r) {

		DMERR("%s: dm_bitset_cursor_skip for dirty failed", __func__);

		dm_bitset_cursor_end(&cmd->dirty_cursor);

		return r;

	}

	while (begin != end) {

		/*

		 * We assume that unmapped blocks have their dirty bit

		 * cleared.

		 */

		r = dm_bitset_test_bit(&cmd->dirty_info, cmd->dirty_root,

				       from_cblock(begin), &cmd->dirty_root, &dirty_flag);

		if (r)

			return r;

		dirty_flag = dm_bitset_cursor_get_value(&cmd->dirty_cursor);

		if (dirty_flag) {

			DMERR("cache block %llu is dirty",

			DMERR("%s: cache block %llu is dirty", __func__,

			      (unsigned long long) from_cblock(begin));

			dm_bitset_cursor_end(&cmd->dirty_cursor);

			*result = false;

			return 0;

		}

		r = dm_bitset_cursor_next(&cmd->dirty_cursor);

		if (r) {

			DMERR("%s: dm_bitset_cursor_next for dirty failed", __func__);

			dm_bitset_cursor_end(&cmd->dirty_cursor);

			return r;

		}

		begin = to_cblock(from_cblock(begin) + 1);

	}

	dm_bitset_cursor_end(&cmd->dirty_cursor);

	return 0;

}