MPI_Sendrecv_replace: correctly handle large data

Because MPI_Sendrecv_replace() uses PMPI_Sendrecv() with MPI_PACKED
under the hood, the data to be exchanged size = MPI_Type_size(datatype) * count
must fit in a signed integer.
Otherwise, PMPI_Sendrecv()
 - fails with an error message if (int)size < 0
 - silently truncate the data if (int)size >= 0

Refs. open-mpi/ompi#8862

Thanks Jakub Benda for reporting this issue and suggesting a fix.

Signed-off-by: Gilles Gouaillardet <gilles@rist.or.jp>