LU-6384 mdt: propagate find errors in mdt_fid2path()

LU-6384 mdt: propagate find errors in mdt_fid2path()

In mdt_fid2path() propagate the specific error from mdt_object_find()
rather than returning -EINVAL.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: Ib09f3741f95c0f3484f9a7839e31c583ecc34761
Reviewed-on: http://review.whamcloud.com/14108
Tested-by: Jenkins
Reviewed-by: frank zago <fzago@cray.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>