staging: erofs: rename source files for better understanding

Keep in line with erofs-outofstaging patchset as well, see
https://lore.kernel.org/linux-fsdevel/20190725095658.155779-1-gaoxiang25@huawei.com/

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Link: https://lore.kernel.org/r/20190731155752.210602-3-gaoxiang25@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>