merge branches/starpu-1.2@18468: mpi/examples/user_datatype/user_datatype.c: add a barrier before waiting for tasks completion

mpi/examples/user_datatype/user_datatype.c

@@ -99,6 +99,7 @@ int main(int argc, char **argv)
 		starpu_mpi_isend_detached(handle0, 0, 20, MPI_COMM_WORLD, NULL, NULL);
 	}
 
+	starpu_mpi_barrier(MPI_COMM_WORLD);
 	starpu_mpi_wait_for_all(MPI_COMM_WORLD);
 
 	starpu_mpi_datatype_unregister(handle0);