Disabling writeback caching

This patch disables writeback caching. Whilst writeback caching gives us better performance, it introduces problems as well. Firstly, it seems that it makes fuse_lowlevel_notify_inval_* useless. Secondly, it is not recommended for use with network filesystems, of which there are many KIO Slaves that interact with them: https://marc.info/?l=linux-fsdevel&m=150592100207655&w=2