LU-930 doc: remove llapi_layout_t from man pages

LU-930 doc: remove llapi_layout_t from man pages

Remove the use of the non-existent llapi_layout_t from llapi_layout.7
and llapi_layout_alloc.3 and replace with struct llapi_layout.

Test-Parameters: trivial
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: Icb3753038677ce95a35ed8f3b8266315553ebbe5
Reviewed-on: https://review.whamcloud.com/26698
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>