* mount: expose `-fuse.maxBackground` flag (closes#9258)
The Linux FUSE driver caps in-flight async requests via
`/sys/fs/fuse/connections/<id>/max_background` (and a derived
`congestion_threshold = 3/4 * max_background`). Heavy upload workloads
need this raised, but the cap currently lives only in `/sys`, so it
resets on reboot/remount.
`weed mount` was hardcoding `MaxBackground: 128`. Promote it to a flag,
default unchanged. Setting `-fuse.maxBackground=2048` reproduces the
manual `echo 2048 > .../max_background` (and gives 1536 for
congestion_threshold automatically) persistently across remounts.
`congestion_threshold` is not exposed as a separate flag because
go-fuse derives it as 3/4 of MaxBackground in InitOut and offers no
hook to override; users wanting a different ratio can still write
/sys/fs/fuse/connections/<id>/congestion_threshold post-mount.
* mount: add `-fuse.congestionThreshold` flag, bump go-fuse to v2.9.3
go-fuse v2.9.3 exposes CongestionThreshold as a separate MountOption,
so we can now let users override the kernel's default 3/4-of-max_background
ratio at mount time instead of having to write
/sys/fs/fuse/connections/<id>/congestion_threshold post-mount on every
remount/reboot.
Default 0 preserves existing behavior (kernel derives it as
3/4 * max_background). Non-zero is sent to the kernel verbatim; the
kernel clamps it to max_background if higher.
Chris Lu
735e94f6ba