LU-4976 osp: add comments for osp_md_object functions

LU-4976 osp: add comments for osp_md_object functions

1. Add introductory comments for osp_md_object.c.
2. Add comments for each function in osp_md_object.c.
3. Remove a few useless functions.
4. fix a typo in the error handler of osp_md_index_lookup.

Signed-off-by: wang di <di.wang@intel.com>
Change-Id: I4ed8372f667f66e9ea15140ea39cc44a75f5abd6
Reviewed-on: http://review.whamcloud.com/10359
Tested-by: Jenkins
Reviewed-by: Ned Bass <bass6@llnl.gov>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>