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

io_uring/napi.c

@@ -222,6 +222,8 @@ int io_register_napi(struct io_ring_ctx *ctx, void __user *arg)
 	};
 	struct io_uring_napi napi;
 
+	if (ctx->flags & IORING_SETUP_IOPOLL)
+		return -EINVAL;
 	if (copy_from_user(&napi, arg, sizeof(napi)))
 		return -EFAULT;
 	if (napi.pad[0] || napi.pad[1] || napi.pad[2] || napi.resv)