staging: lustre: nfs: don't panic NFS server if MDS fails to find FID

When MDS fails to retrive the parent's fid, we'd handle it without
crashing the NFS server.

Signed-off-by: Bobi Jam <bobijam.xu@intel.com>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-3952
Reviewed-on: http://review.whamcloud.com/8459


Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: wangdi <di.wang@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>