LU-9165 pfl: MDS handling of write intent IT_LAYOUT RPC

LU-9165 pfl: MDS handling of write intent IT_LAYOUT RPC

* Instantiate the OST objects of the 1st component of the PFL when its
  data are being created or its layout being set.

* Hence component_add won't instantiate OST objects, the instantiation
  is relayed to later write/truncate access.

* When the client tries to write beyond OST-object-instantiated extent
  range, it will issue an IT_LAYOUT intent lock RPC, indicating the
  write range, MDS needs to handle this RPC to allocate all OST
  objects of components covering the range.

* MDS also needs to handle the replay of write intent IT_LAYOUT RPC,
  which contains the current layout objects which the client knows, so
  that MDS won't initialize those objects again.

Reviewed-on: https://review.whamcloud.com/25717

Signed-off-by: Bobi Jam <bobijam.xu@intel.com>
Change-Id: Idcb242af55788dd8d7f01a6ecec4fbe508375512
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>