vchoi_fork

[svn-r5418] Purpose:
    Code cleanup/Performance enhancement

Description:
    The code to extend the file size in H5FD_mpio_flush is getting run even when
    the file size doesn't change.  Also, it's sort of sidestepping MPI-I/O when
    extending the file, instead of using MPI-I/O features to set the file's size.

Solution:
    Only extend the file's size when the allocated size has increased.  Also
    use MPI_File_set_size() to change the file's size.

Platforms tested:
    IRIX64 6.5 (modi4)