Improve MPI perf by keeping region threading for buffer packing.
Improve MPI perf by keeping region threading for buffer packing.

Before this change, the max number of threads was used for buffer packing.
This caused OpenMP to switch threading schemes on every time-step.
Now, the MPI packing just uses the region threads w/o changing them.

01org/yask,01org/yask,01org/yask,01org/yask