staging/lustre: 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.

Reviewed-on: http://review.whamcloud.com/8347
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-4231


Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Signed-off-by: Dmitry Eremin <dmitry.eremin@intel.com>
Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Acked-by: Jeff Layton <jlayton@poochiereds.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>