mount: expose -fuse.maxBackground and -fuse.congestionThreshold flags (closes #9258) (#9268)

* 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.

weed/command/mount_std.go

@@ -242,11 +242,21 @@ func RunMount(option *MountOptions, umask os.FileMode) bool {
 		fsName = "fuse"
 	}
 
+	maxBackground := 128
+	if option.fuseMaxBackground != nil && *option.fuseMaxBackground > 0 {
+		maxBackground = *option.fuseMaxBackground
+	}
+	congestionThreshold := 0
+	if option.fuseCongestionThreshold != nil && *option.fuseCongestionThreshold > 0 {
+		congestionThreshold = *option.fuseCongestionThreshold
+	}
+
 	// mount fuse
 	fuseMountOptions := &fuse.MountOptions{
 		AllowOther:               *option.allowOthers,
 		Options:                  option.extraOptions,
-		MaxBackground:            128,
+		MaxBackground:            maxBackground,
+		CongestionThreshold:      congestionThreshold,
 		MaxWrite:                 1024 * 1024 * 2,
 		MaxReadAhead:             1024 * 1024 * 2,
 		IgnoreSecurityLabels:     false,