LU-8998 lod: accomodate to composite layout

LU-8998 lod: accomodate to composite layout

Modify the LOD to make it support the composite layout:

- Introduced lod_layout_component structure to represent each
  individual layout component, and added ldo_comp_entries and
  ldo_comp_cnt in the lod_object structure to hold the striping
  information for composite layout.

- Object allocation code is adjusted to not only check the used
  OSTs in this round of allocation, but also the used OSTs in
  the existing layout components.

- Support special xattr names to add/set/delete layout component(s).

- Store default layout xattr on the filesystem root inode.

Reviewed-on: https://review.whamcloud.com/24823

Change-Id: I73736df1cd529a3b0e32b2bb6a09b5c436e08c86
Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Signed-off-by: Bobi Jam <bobijam.xu@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>