kbfsfuse: refuses to mount over bcachefs

Describe the bug

To Reproduce
Steps to reproduce the behavior:

1. Set up a Linux system with /home on a bcachefs filesystem
2. Run kbfsfuse ~/Keybase at the console

Expected behavior

Keybase filestore is available at ~/Keybase

Screenshots

❯ uname -a  
Linux gwydion 6.15.10-xanmod1 #1-NixOS SMP PREEMPT_DYNAMIC Tue Jan  1 00:00:00 UTC 1980 x86_64 GNU/Linux


❯ kbfsfuse ~/Keybase   
▶ INFO KBFS version 6.5.1
▶ DEBUG Connecting to socket with dialFiles=[/run/user/5001/keybase/keybased.sock], bindFiles=/run/user/5001/keybase/keybased.sock
▶ WARNING adding NotifyService protocol
▶ DEBUG + GetSocket
▶ DEBUG + GetSocket
▶ DEBUG - GetSocket -> ok [time=791ns]
▶ DEBUG - GetSocket -> ok [time=691ns]
▶ DEBUG | empty socket wrapper; need a new one
▶ DEBUG + SocketInfo#dialSocket(unix:/run/user/5001/keybase/keybased.sock)
▶ DEBUG | net.Dial(unix:/run/user/5001/keybase/keybased.sock)
▶ DEBUG - SocketInfo#dialSocket(unix:/run/user/5001/keybase/keybased.sock) -> ok [time=50.736µs]
▶ DEBUG | DialSocket -> ok
▶ DEBUG + GetSocket
▶ DEBUG - GetSocket -> ok [time=2.465µs]
▶ INFO OnConnect called with a new connection
▶ INFO authentication successful; ping interval: 30s
2025/08/28 14:28:15 mount helper error: /nix/store/k5lxhm3yqqad0kyvmnzrhci93g4qhkiy-fuse-2.9.9-bin/bin/fusermount: mounting over filesystem type 0xca451a4e is forbidden
2025/08/28 14:28:15 mount helper error: /nix/store/k5lxhm3yqqad0kyvmnzrhci93g4qhkiy-fuse-2.9.9-bin/bin/fusermount: mounting over filesystem type 0xca451a4e is forbidden
KBFS failed to FUSE mount at /home/jez/Keybase: fusermount: exit status 1
▶ DEBUG removing stale socket file: /run/user/5001/keybase/kbfsd.sock
▶ DEBUG + SocketInfo#BindToSocket(unix:/run/user/5001/keybase/kbfsd.sock)
▶ INFO | net.Listen on unix:/run/user/5001/keybase/kbfsd.sock
▶ DEBUG - SocketInfo#BindToSocket(unix:/run/user/5001/keybase/kbfsd.sock) -> ok [time=424.863µs]
▶ INFO Unmounting
▶ INFO Ignoring mount-not-found fusermount error
▶ WARNING KBFS failed to FUSE mount at /home/jez/Keybase: fusermount: exit status 1
▶ ERROR Mounting the filesystem failed: fusermount: exit status 1

Well I guess I'll put the number 345567 back in again...

Start with the kbfsfuse mount path and reproduce the failure on a Linux system whose home filesystem is bcachefs. Compare the fusermount error with successful mounts on other filesystem types; done means kbfsfuse mounts at ~/Keybase and the Keybase filestore is available there.