vfs_busy — marks a
    mount point as busy
The
    vfs_busy()
    function marks a mount point as busy by incrementing the reference count of
    a mount point. It also delays unmounting by sleeping on
    mp if the MNTK_UNMOUNT flag is
    set in mp->mnt_kern_flag and the
    MBF_NOWAIT flag is
    not set.
Its arguments are:
  - mp
 
  - The mount point to busy.
 
  - flags
 
  - Flags controlling how
      
vfs_busy()
      should act.
    
      MBF_NOWAIT 
      - do not sleep if 
MNTK_UNMOUNT is set. 
      MBF_MNTLSTLOCK 
      - drop the mountlist_mtx in the critical path.
 
    
   
A 0 value is returned on success. If the mount point is being
    unmounted and MBF_NOWAIT flag is specified ENOENT
    will be returned.
  - [
ENOENT] 
  - The mount point is being unmounted (
MNTK_UNMOUNT
      is set).