Disable fsync and fdatasync in SafeHardlinkOps FUSE filesystem

A lot of integration commands run fsync in the expectation that they are being run as part of a package manager-driven system upgrade.

This slows down those commands, needlessly in the case of buildstream because we don't need to worry about the sandbox being in an inconsistent state due to machine failure. In our case, either the build succeeded and has been committed to the cache, or something went wrong and we will have to start from scratch.

We could thus potentially speed up integration-commands by making fsync() and fdatasync() into a no-op our FUSE layer.