io_uring: don't delay iopoll'ed req completion

->iopoll() may have completed current request, but instead of reaping
it, io_do_iopoll() just continues with the next request in the list.
As a result it can leave just polled and completed request in the list
up until next syscall. Even outer loop in io_iopoll_getevents() doesn't
help the situation.

E.g. poll_list: req0 -> req1
If req0->iopoll() completed both requests, and @min<=1,
then @req0 will be left behind.

Check whether a req was completed after ->iopoll().

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>