LU-10437 lod: clear layout header when generating layout

LU-10437 lod: clear layout header when generating layout

LOD needs to clear layout header otherwise the lcm_flags and
lcm_padding will be random data, which will create issues when
those fields are used by future module.

It already confused FLR because it uses lcm_flags and mirror_count
to do sanity check.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: If9511e6691144debd51ccab575ef4479d0c9b865
Reviewed-on: https://review.whamcloud.com/30785
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Tested-by: Maloo <hpdd-maloo@intel.com>