LU-4231 llite: proper support of NFS anonymous dentries

LU-4231 llite: proper support of NFS anonymous dentries

NFS can ask to encode dentries that are not connected to the root.
The fix check for parent is NULL and encode a file handle accordingly.

Signed-off-by: Dmitry Eremin <dmitry.eremin@intel.com>
Change-Id: Idba91fd4bca4f26a37fd9bc76a340d2fbf557c9e
Reviewed-on: http://review.whamcloud.com/8347
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Jenkins
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>