So much workloads want to merge a several files in the one. Long time ago - lustre had a 'JOIN file' feature addressed it but it removed.

Once PFL exist it might be used to implement file join by adding components to a file with non-zero offset, but original PFL implementation has a bug/feature - which required start object from zero offset.

This prevents file join to use existing objects, and also caused troubles with large files on the ldiskfs backend.

Let's implement both features.