LU-1452 scrub: OI scrub skips uninitialized groups

LU-1452 scrub: OI scrub skips uninitialized groups

If the ldiskfs group descriptor is marked as LDISKFS_BG_INODE_UNINIT,
then means that the inodes in such group have never been initialized,
so the otable based iterator can skip this group directly to speed up
the scanning.

If the iteration position reaches the unused inodes area in the
group descriptor (indicated by bg_itable_unused), then skip the
rest inodes in this group to reduce the scanning time.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: Ie8a2eb1269d288865ce51d40e211e3db54d062af
Reviewed-on: http://review.whamcloud.com/12737
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>