hdf5_msb

Fix the FSM bug when setting the FSM threshold to a non-default value.
Check for smaller or larger section size after merging and shrinking a section,
for this case is the section that is smaller than threshold (see H5MF_xfree() in H5MF.c).
It is possible for the section to be smaller after merging/shrinking (see H5MF__sect_large_shrink()
in H5MFsection.c).