malloc: Use memsize instead of CHUNK_AVAILABLE_SIZE

This is a pure refactoring change that does not affect behaviour.

The CHUNK_AVAILABLE_SIZE name was unclear, the memsize name tries to
follow the existing convention of mem denoting the allocation that is
handed out to the user, while chunk is its internally used container.

The user owned memory for a given chunk starts at chunk2mem(p) and
the size is memsize(p).  It is not valid to use on dumped heap chunks.

Moved the definition next to other chunk and mem related macros.

Reviewed-by: DJ Delorie <dj@redhat.com>