io_uring: simplify io_uring_cmd return

We don't have to return error code from an op handler back to core
io_uring, so once io_uring_cmd() sets the results and handles errors we
can juts return IOU_OK and simplify the code.

Note, only valid with e0b23d99

 ("io_uring: optimise ltimeout for
inline execution"), there was a problem with iopoll before.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/8eae2be5b2a49236cd5f1dadbd1aa5730e9e2d4f.1721819383.git.asml.silence@gmail.com


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