From: Maneesh Soni <maneesh@in.ibm.com>

- hpfs_unlink() can race with lockless d_lookup(), as we can have situations
  where d_lookup() has successfully looked-up a dentry and at the sametime
  hpfs_unlink()--->d_drop() has dropped it. Taking the per dentry lock
  before checking the d_count in hpfs_unlink() solves this race condition.