Fix getting starpu_mpi maximum tag

This fixes using starpu trunk at all :)

The StarPU-mpi API for attribute is very different from the MPI one.

@furmento : do we really want to diverge so much from the MPI standard concerning retrieving attributes? AIUI the idea of the standard is that it writes the address of the value, not the value. Of course we can't respect the standard for MPI_UB since the standard says int, and we want to be able to express more than int, but at least the principle (using a pointer) could be coherent with the standard I guess?