block: don't make REQ_POLLED imply REQ_NOWAIT

Normally these two flags do go together, as the issuer of polled IO
generally cannot wait for resources that will get freed as part of IO
completion. This is because that very task is the one that will complete
the request and free those resources, hence that would introduce a
deadlock.

But it is possible to have someone else issue the polled IO, eg via
io_uring if the request is punted to io-wq. For that case, it's fine to
have the task block on IO submission, as it is not the same task that
will be completing the IO.

It's completely up to the caller to ask for both polled and nowait IO
separately! If we don't allow polled IO where IOCB_NOWAIT isn't set in
the kiocb, then we can run into repeated -EAGAIN submissions and not
make any progress.

Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>