[$] FUSE passthrough for file I/O

There are some filesystems that use the Filesystemin Userspace (FUSE) framework but only to provide a different view ofan underlying filesystem, such as different filemetadata, a changed directory hierarchy, or other changes of that sort.The read-only filteredfilesystem, which simply filters the view of which filesare available, is one example; the file data could come directly from theunderlying filesystem, but currently needs to traverse the FUSE user-space serverprocess. Finding a way to bypass the server, so that the file I/O operations godirectly from the application to the underlying filesystem would be beneficial. Ina filesystem session at the 2023 Linux Storage,Filesystem, Memory-Management and BPF Summit, Miklos Szeredi wanted to exploredifferent options for adding such a mechanism, which was referred to asa "FUSE passthrough"-though "bypass" might be a better alternative.