io_uring: don't allow netpolling with SETUP_IOPOLL

IORING_SETUP_IOPOLL rings don't have any netpoll handling, let's fail
attempts to register netpolling in this case, there might be people who
will mix up IOPOLL and netpoll.

Cc: stable@vger.kernel.org
Fixes: ef1186c1 ("io_uring: add register/unregister napi function")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/1e7553aee0a8ae4edec6742cd6dd0c1e6914fba8.1721819383.git.asml.silence@gmail.com


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