LU-8998 llapi: add LLAPI_LAYOUT_COMP_USE_PREV

LU-8998 llapi: add LLAPI_LAYOUT_COMP_USE_PREV

Add LLAPI_LAYOUT_COMP_USE_PREV to be able to iterate through
components in reverse order.

Add a test case to llapi_layout_test.c to exercise COMP_USE_LAST
and COMP_USE_PREV options.

Improve description of component ID in llapi_layout_comp_id_get.3
to indicate that the component ID does not imply ordering or other
semantics, and is just a numeric identifier for each component.

Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: I21f78e575c2429ef927c8c2fc50bf150f59cab07
Reviewed-on: https://review.whamcloud.com/26484
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>