malloc: Correct size computation in realloc for dumped fake mmapped chunks

malloc: Correct size computation in realloc for dumped fake mmapped chunks

For regular mmapped chunks there are two size fields (hence a reduction
by 2 * SIZE_SZ bytes), but for fake chunks, we only have one size field,
so we need to subtract SIZE_SZ bytes.

This was initially reported as Emacs bug 23726.