LU-957 scrub: OSD layer cleanup for OI scrub

LU-957 scrub: OSD layer cleanup for OI scrub

1) Abstract some common functions in OSD layer.
2) Call lu_object_put() in osd_object_find() under failure case.
3) Do not release buffer_head until all the needed contents have
   been accessed in osd_oi_index_create_one().

Signed-off-by: Fan Yong <yong.fan@whamcloud.com>
Change-Id: I3e6880271ee73ec2c4638b02c7e9c55fc827bdf3
Reviewed-on: http://review.whamcloud.com/2551
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>