mount2: add --allow-idmap to advertise FUSE_ALLOW_IDMAP

Lets the kernel id-map a mount2 mount into a user namespace
(e.g. Kubernetes pods with hostUsers: false). Off by default;
requires Linux 6.12+ and implies default_permissions.

cmd/mount2/mount.go

@@ -33,6 +33,7 @@ func mountOptions(fsys *FS, f fs.Fs, opt *mountlib.Options) (mountOpts *fuse.Mou
 		MaxReadAhead:       int(fsys.opt.MaxReadAhead),
 		MaxWrite:           1024 * 1024, // Linux v4.20+ caps requests at 1 MiB
 		DisableReadDirPlus: true,
+		IDMappedMount:      opt.AllowIDMap,
 
 		// RememberInodes: true,
 		// SingleThreaded: true,