Callback for H5Pset_append_flush() is never invoked by library

[qkoziol]

Describe the bug The function pointer gets stored, but is never called.

Expected behavior Call it according to the spec for H5Pset_append_flush()

Platform (please complete the following information) All

Additional context It is semi-tested in test/swmr.c: the set/get of the property is checked, but there's no actual validation that the callback is invoked.