LU-5565 osd-ldiskfs: handle non-existing objects

In the DT API method implementations in lustre/osd/osd_ldiskfs.c
replace assertions that the object exists (or does not exist) with
appropriate error returns.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: If346861eb352efaf0b90a8e7778e8fbb3cf16bf0
Reviewed-on: http://review.whamcloud.com/12608
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>