LU-2842 mdt: Do not lbug on invalid fid entry in name

Instead of crashing MDS when the name exists but has invalid fid,
we should just return error and let lfsck deal with it later.

Change-Id: Iea19dac75e9dc75c43a18efa757fc1c5e4d1743e
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Reviewed-on: http://review.whamcloud.com/7617
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: wangdi <di.wang@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>