staging: erofs: avoid magic constants when initializing clusterbits

Currently erofs only supports clustersize == blocksize.
and clustersize == 2^n * blocksize will be supported in the future.

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>