LU-8998 lov: add composite layout unpacking

LU-8998 lov: add composite layout unpacking

Update struct lov_stripe_md to accommodate composite layouts. Add
methods to unpack composite layouts.

Reviewed-on: https://review.whamcloud.com/24849

Change-Id: Ife0b2ae7671dd0ec0020a1da04e7261ef9f7e5f3
Signed-off-by: John L. Hammond <john.hammond@intel.com>
Signed-off-by: Bobi Jam <bobijam.xu@intel.com>
Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>