LU-3285 llite: check layout size after cl_object_layout_get

LU-3285 llite: check layout size after cl_object_layout_get

Check that layout size is not zero before doing flush for
DoM object. Also patch does that:
- initializes all values in cl_layout explicitly
  in lov_object_layout_get().
- changes cl_dom_comp_size to u64 for future needs, e.g.
  importing an existing filesystem image directly.

Test-Parameters: mdssizegb=20 testlist=sanity-dom,dom-performance
Signed-off-by: Mikhal Pershin <mike.pershin@intel.com>
Change-Id: Ic6ff06ad4022f85686fda238eae34ad38ff2dcd0
Reviewed-on: https://review.whamcloud.com/29810
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>