If using Linux 6.9 or above and enabled (passthrough=true) files
opened or created will use the FUSE passthrough feature.
If direct-io=true / cache.files=off it will override passthrough. If
direct-io-allow-mmap is enabled only mmap will passthrough.
HANDLE_KILLPRIV and V2 are enabled now by default to remove the
kernel's need to issue getattr and getxattr requests.
moveonenospc will not work when leveraging passthrough.
After numerous tests it was found the splice features were at best
the same performance as standard IO and at worse actually slower.
To simplify the code all splice features are removed.
Linux 4.20 and above allow setting the number of pages per FUSE message
upto 256 (4K * 256 = 1MiB). This can greatly increase read and write
speeds depending on the workload.