Weird usages of sentinel_ok()

In [1], Matthias is proposing to introduce a new system memory context
type that basically ties palloc directly to malloc. While reviewing
that patch, I noticed it copied a usage of sentinel_ok() that deviates
from the norm. Technically it's fine, as sentinel_ok()'s first
parameter is a void pointer, so passing the MemoryChunk rather than
what palloc() would return is fine, provided you add on the byte
difference to the MemoryChunk. I felt the need to mention the
weirdness during the review, and since people clearly copy a load of
code from an existing context type when adding new ones, it means that
pattern is going to be copied again when the next person adds a new
allocator.

I thought it might be good to adjust this at the source to prevent that.

Anyone disagree?

Patch attached.

David

[1] https://postgr.es/m/CAApHDvp_-zh5cdR_aNaUCXGHeddWC0b1a5kiJpC1cKdG0vx6DA@mail.gmail.com