LU-1866 osd: ancillary work for initial OI scrub

1) Abstract some general interfaces that can be shared by common
   OSD operations, by normal OI scrub, and by initial OI scrub.

2) Mark the object as "LDISKFS_STATE_LUSTRE_NO_OI" if it is not
   unnecessary to add OI mapping for the object, such as backend
   local root, OI directories/files, and others created before OI
   files created.

3) Do not check fld in OI scurb, which may be not ready yet.

4) Reorganize OI scrub code to be more readable.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: Ife8168df2454437ad18550b73d26f7372c21776f
Reviewed-on: http://review.whamcloud.com/4902
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Hudson
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>