LU-3059 lu: shrink lu_object_header by 8 bytes on x86_64

LU-3059 lu: shrink lu_object_header by 8 bytes on x86_64

Locate the loh_flags and loh_ref fields together in lu_object_header
to avoid holes and shrink the structure by 8 bytes.

Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: I36b6132a819b6557d6d1688adb3e1f7d282d22b5
Reviewed-on: http://review.whamcloud.com/9185
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Liang Zhen <liang.zhen@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>