In cb_dumphdr() we were calling buf_write() with di->priv as the
pointer to a dumperinfo instead of di. A brainfart, surely. This
bug went unnoticed for all this time because the pointer is only
used by buf_write() when it can write a completely filled buffer
to the dump device. This depends on the number of memory chunks
that needs to be dumped. This has apparently been low enough that
it has never happened up until this point.