LU-11212 lod: preserve mirror ID on mirror extension

LU-11212 lod: preserve mirror ID on mirror extension

When merging/expanding existing mirrors of a FLR file, we need keep
its existing mirror's mirror ID.

Signed-off-by: Bobi Jam <bobijam.xu@intel.com>
Change-Id: If139076c37c33bb1a330e1a5e997f8f56015fd9a
Reviewed-on: https://review.whamcloud.com/32938
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>