Ensure that prep handlers always initialize sr->done_io before any
potential failure conditions, and with that, we now it's always been
set even for the failure case.

With that, we don't need to use the REQ_F_PARTIAL_IO flag to gate on that.
Additionally, we should not overwrite req->cqe.res unless sr->done_io is
actually positive.
Signed-off-by: Jens Axboe <axboe@kernel.dk>