Adjust Valgrind macro usage to protect chunk headers

Adjust Valgrind macro usage to protect chunk headers

Prior to this commit we only ever protected MemoryChunk's requested_size
field with Valgrind NOACCESS.  This means that if the hdrmask field is
ever accessed accidentally then we're not going to get any warnings from
Valgrind about it.  Valgrind would have warned us about the problem fixed
in 92957ed98 had we already been doing this.

Per suggestion from Tom Lane

Reviewed-by: Richard Guo
Discussion: https://postgr.es/m/1650235.1672694719@sss.pgh.pa.us
Discussion: https://postgr.es/m/CAApHDvr=FZNGbj252Z6M9BSFKoq6BMxgkQ2yEAGUYoo7RquqZg@mail.gmail.com