This work tracked the MDS handling of PFL layout object initialization.

• Initialize the OST objects of the 1st component of the PFL when its data are being created or its layout being set.
• When the client tries to write beyond OST-object-initialized extent range, it will issue an IT_LAYOUT_MOD intent lock RPC, indicating the end offset of the write range, MDS needs to handle this RPC to allocate all OST objects of components covering this range.
• MDS also needs to handle the replay of IT_LAYOUT_MOD RPC, which contains the current layout objects which the client knows, so that MDS won't allocate those OST objects again.