[svn-r5368] Use the correct type in the segment calls - it must be an MPI_Aint, not an MPIDI_msg_sz_t, particularly when the size of MPI_Aint is changed to match MPI_Offset (the segment calls specify an MPI_Aint at the last argument, for example.  This is part of the changes needed to make attributes work properly when --with-aint-size=8 is selected