hdf5_msb

[svn-r16523] Purpose: Fix bugs related to H5Dset_extent and fill values

Description:
In some situations it was possible for the fill value to not be written to parts
of a chunked dataset, particularly when extending and/or shrinking.  Prior to
the fix for the chunk cache (1015) these bugs would have been exceedingly rare.

Tested: jam, smirom, linew (h5committest)