__d_drop — drop a dentry
| void __d_drop ( | struct dentry * dentry ); | 
   d_drop unhashes the entry from the parent dentry hashes, so that it won't
   be found through a VFS lookup any more. Note that this is different from
   deleting the dentry - d_delete will try to mark the dentry negative if
   possible, giving a successful _negative_ lookup, while d_drop will
   just make the cache lookup fail.
   
   d_drop is used mainly for stuff that wants to invalidate a dentry for some
   reason (NFS timeouts or autofs deletes).
   
__d_drop requires dentry->d_lock.