LU-9357 pfl: should inherit pool from previous layout comp

LU-9357 pfl: should inherit pool from previous layout comp

llapi_layout_comp_add() should inherit pool from previous component
layout so that user doesn't need to specify the same pool name
everytime a new component is added if necessary.

Signed-off-by: Bobi Jam <bobijam.xu@intel.com>
Change-Id: I5437112d9eb51b7c4a3e5e67b65302df15eca681
Reviewed-on: https://review.whamcloud.com/26750
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>