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madmpi: fix MPI_Reduce_scatter

Philippe SWARTVAGHER requested to merge pswartva/pm2:fix-mpi_reduce_scatter into master

Hi,

MPI_Reduce_scatter has a bug, discovered while executing this example (line 42 of the example is wrong, but that's another story...).

Compare the result with MadMPI:

[MPI process 0] The sum I received is 12.
[MPI process 1] The sum I received are 18 and 21.
[MPI process 2] The sum I received is 18.

and with OpenMPI:

[MPI process 0] The sum I received is 12.
[MPI process 1] The sum I received are 15 and 18.
[MPI process 2] The sum I received is 21.

With MadMPI, process 1 receives 18 and 21 while it should receive 15 and 21.

This MR fixes this bug.

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