io_uring/rw: don't lose partial IO result on fail

A partially done read/write may end up in io_req_complete_failed() and
loose the result, make sure we return the number of bytes processed.

Cc: stable@vger.kernel.org
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/05e0879c226bcd53b441bf92868eadd4bf04e2fc.1663668091.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>