kernel - Fix pty clone vnode leak

kernel - Fix pty clone vnode leak

* Any cloned devfs device (vn, tap, ptmx primarily) was leaving an extra
  vref on the related vnode, preventing the vnode from being recycled.

  This was due to devfs called vop_stdopen() but forgetting that
  vop_stdopen() will load the file pointer (fp), which devfs was also
  loading.  All devfs really needs to do is to replace fp->f_ops.

Reported-by: swildner