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

- nfs_unlink() can race with lockless d_lookup() as d_lookup() can
  successfully lookup a dentry for which nfs_unlink() can assume that no one
  else is using and can go ahead and do nfs_safe_remove() on it. By using
  per dentry lock, it is solved as we d_lookup() will fail the lookup for
  unhashed dentries.