LU-9972 osd: cache OI mapping in dt_declare_ref_add

LU-9972 osd: cache OI mapping in dt_declare_ref_add

so that subsequent calls to manipulate /PENDING don't need to
consult with OI.
use OIC in osd_index_ea_delete() to optimize FLDB lookups.

Change-Id: I779bbddb429b577aecf1ad092d74d0802e43d567
Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-on: https://review.whamcloud.com/29709
Tested-by: Jenkins
Reviewed-by: Fan Yong <fan.yong@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>