coccinelle: add SIZEOF() macro to work-around sizeof(*private)

coccinelle: add SIZEOF() macro to work-around sizeof(*private)

We have code like `size_t max_size = sizeof(*private)` in three
places. This is evaluated at compile time so its safe to use. However
the new pointer-deref checker in coccinelle is not smart enough to know
this and will flag those as errors. To avoid these false positives
we have some options:
1. Reorder so that we do:
```C
size_t max_size = 0;
assert(private);
max_size = sizeof(*private);
```
2. Use something like `size_t max_size = sizeof(*ASSERT_PTR(private));`
3. Place the assert before the declaration
4. Workaround coccinelle via SIZEOF(*private) that we can then hide
   via parsing_hacks.h
5. Fix coccinelle (OCaml, hard)
6. ... somehting I missed?

None of these is very appealing. I went for (4) but happy about
suggestions.